Linux 5.6 compat: fix fs_parse API mismatch

Added m4 macro to check fs_parse API signature and wrappers.  Before 
5.6, fs_parse() took a struct fs_parameter_description which wraps
the parameter specs with name and enum pointers. From 5.6, the 
description struct was removed and fs_parse() accepts the 
fs_parameter_spec directly.

Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: tiehexue <tiehexue@hotmail.com>
Closes #18585

config/kernel-fs-parse.m4

@@ -0,0 +1,34 @@
+dnl # SPDX-License-Identifier: CDDL-1.0
+dnl #
+dnl # 5.6 API change
+dnl # Before 5.6, fs_parse() took a struct fs_parameter_description
+dnl # which wraps the parameter specs with name and enum pointers. From 5.6,
+dnl # the description struct was removed and fs_parse() accepts the
+dnl # fs_parameter_spec directly.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_SRC_FS_PARSE], [
+	ZFS_LINUX_TEST_SRC([fs_parse], [
+		#include <linux/fs_context.h>
+		#include <linux/fs_parser.h>
+	],[
+		static const struct fs_parameter_spec specs[] = {
+			{}
+		};
+		int test __attribute__ ((unused));
+		struct fs_context *fc __attribute__ ((unused)) = NULL;
+		struct fs_parameter param __attribute__ ((unused));
+		struct fs_parse_result result __attribute__ ((unused));
+		test = fs_parse(fc, specs, &param, &result);
+	])
+])
+
+AC_DEFUN([ZFS_AC_KERNEL_FS_PARSE], [
+	AC_MSG_CHECKING([whether fs_parse() takes fs_parameter_spec directly])
+	ZFS_LINUX_TEST_RESULT([fs_parse], [
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_FS_PARSE_TAKES_SPEC, 1,
+		    [fs_parse() takes fs_parameter_spec directly])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -78,6 +78,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC_TRUNCATE_SETSIZE
 	ZFS_AC_KERNEL_SRC_SECURITY_INODE
 	ZFS_AC_KERNEL_SRC_FS_CONTEXT
+	ZFS_AC_KERNEL_SRC_FS_PARSE
 	ZFS_AC_KERNEL_SRC_SB_DYING
 	ZFS_AC_KERNEL_SRC_SET_NLINK
 	ZFS_AC_KERNEL_SRC_SGET
@@ -201,6 +202,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL_TRUNCATE_SETSIZE
 	ZFS_AC_KERNEL_SECURITY_INODE
 	ZFS_AC_KERNEL_FS_CONTEXT
+	ZFS_AC_KERNEL_FS_PARSE
 	ZFS_AC_KERNEL_SB_DYING
 	ZFS_AC_KERNEL_SET_NLINK
 	ZFS_AC_KERNEL_SGET

module/os/linux/zfs/zpl_super.c

@@ -613,13 +613,34 @@ static const struct fs_parameter_spec zpl_param_spec[] = {
 	{}
 };
 
+/*
+ * Before 5.6, fs_parse() took a struct fs_parameter_description
+ * which wraps the parameter specs with name and enum pointers. From 5.6,
+ * the description struct was removed and fs_parse() accepts the
+ * fs_parameter_spec directly.
+ */
+static int
+zpl_fs_parse(struct fs_context *fc, struct fs_parameter *param,
+	struct fs_parse_result *result)
+{
+#ifdef HAVE_FS_PARSE_TAKES_SPEC
+	return (fs_parse(fc, zpl_param_spec, param, result));
+#else
+	static const struct fs_parameter_description zpl_param_desc = {
+		.name = "zfs",
+		.specs = zpl_param_spec,
+	};
+	return (fs_parse(fc, &zpl_param_desc, param, result));
+#endif
+}
+
 static int
 zpl_parse_param(struct fs_context *fc, struct fs_parameter *param)
 {
 	vfs_t *vfs = fc->fs_private;
 
 	struct fs_parse_result result;
-	int opt = fs_parse(fc, zpl_param_spec, param, &result);
+	int opt = zpl_fs_parse(fc, param, &result);
 	if (opt == -ENOPARAM) {
 		/*
 		 * Convert unknowns to warnings, to work around the whole
@@ -803,7 +824,7 @@ zpl_parse_monolithic(struct fs_context *fc, void *data)
 
 		/* Check if this is one of our options. */
 		struct fs_parse_result result;
-		int opt = fs_parse(fc, zpl_param_spec, &param, &result);
+		int opt = zpl_fs_parse(fc, &param, &result);
 		if (opt >= 0) {
 			/*
 			 * We already know this one of our options, so a