zpool/zfs: accept --help and -? after a subcommand

Print the short usage instead of "invalid option".

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Christos Longros <chris.longros@gmail.com>
Closes #18541

cmd/zfs/zfs_main.c

@@ -9399,6 +9399,18 @@ main(int argc, char **argv)
 		return (1);
 	}
 
+	/*
+	 * Special case '<subcommand> --help|-?'
+	 */
+	if (argc >= 3 && (strcmp(argv[2], "--help") == 0 ||
+	    strcmp(argv[2], "-?") == 0)) {
+		int idx;
+		if (find_command_idx(cmdname, &idx) == 0) {
+			current_command = &command_table[idx];
+			usage(B_FALSE);
+		}
+	}
+
 	zfs_save_arguments(argc, argv, history_str, sizeof (history_str));
 
 	libzfs_print_on_error(g_zfs, B_TRUE);

cmd/zpool/zpool_main.c

@@ -13878,6 +13878,18 @@ main(int argc, char **argv)
 	if (strcmp(cmdname, "help") == 0)
 		return (zpool_do_help(argc, argv));
 
+	/*
+	 * Special case '<subcommand> --help|-?'
+	 */
+	if (argc >= 3 && (strcmp(argv[2], "--help") == 0 ||
+	    strcmp(argv[2], "-?") == 0)) {
+		int idx;
+		if (find_command_idx(cmdname, &idx) == 0) {
+			current_command = &command_table[idx];
+			usage(B_FALSE);
+		}
+	}
+
 	if ((g_zfs = libzfs_init()) == NULL) {
 		(void) fprintf(stderr, "%s\n", libzfs_error_init(errno));
 		return (1);