LinuxKPI/lindebugfs: stop panicing in lindebugfs, fix simple_read_from_buffer

Trying to use lindebugfs for debugging wirless drivers two issues
became apparent:
(a) a panic in lindebugfs calling a hard coded release function if the
    caller had not provided one.  This seems to be based on assumptions
    that no longer hold up.  Remove the hard coded release function to
    prevent panics.
(b) In LinuxKPI simple_read_from_buffer() would call copy_to_user() but
    buffers weren't setup for this (lindebugfs copies data from its
    own buffer) and then pseudofs will do another copyout to the user
    on this;  remove the copy_to_user() and simply copy the data over
    to the provided buffer; this works for as long as the only consumers
    remain debugfs callers (which currently seems to be the case).
    [the only out-of-tree consumers I am aware off are two drm-kmod
    drivers/gpu/drm/amd/pm/* debugfs functions I cannot test].

Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Tested by:	jfree
Differential Revision: https://reviews.freebsd.org/D45755

sys/compat/lindebugfs/lindebugfs.c

@@ -162,8 +162,6 @@ debugfs_fill(PFS_FILL_ARGS)
 
 	if (d->dm_fops->release)
 		d->dm_fops->release(&vn, &lf);
-	else
-		single_release(&vn, &lf);
 
 	if (rc < 0) {
 #ifdef INVARIANTS

sys/compat/linuxkpi/common/include/linux/fs.h

@@ -353,9 +353,8 @@ static inline ssize_t
 simple_read_from_buffer(void __user *dest, size_t read_size, loff_t *ppos,
     void *orig, size_t buf_size)
 {
-	void *read_pos = ((char *) orig) + *ppos;
+	void *p, *read_pos = ((char *) orig) + *ppos;
 	size_t buf_remain = buf_size - *ppos;
-	ssize_t num_read;
 
 	if (buf_remain < 0 || buf_remain > buf_size)
 		return -EINVAL;
@@ -363,13 +362,18 @@ simple_read_from_buffer(void __user *dest, size_t read_size, loff_t *ppos,
 	if (read_size > buf_remain)
 		read_size = buf_remain;
 
-	/* copy_to_user returns number of bytes NOT read */
-	num_read = read_size - copy_to_user(dest, read_pos, read_size);
-	if (num_read == 0)
-		return -EFAULT;
-	*ppos += num_read;
+	/*
+	 * XXX At time of commit only debugfs consumers could be
+	 * identified.  If others will use this function we may
+	 * have to revise this: normally we would call copy_to_user()
+	 * here but lindebugfs will return the result and the
+	 * copyout is done elsewhere for us.
+	 */
+	p = memcpy(dest, read_pos, read_size);
+	if (p != NULL)
+		*ppos += read_size;
 
-	return (num_read);
+	return (read_size);
 }
 
 MALLOC_DECLARE(M_LSATTR);