btrfs: prop: fix vanished compression property after failed set

The compression property resets to NULL, instead of the old value if we
fail to set the new compression parameter.

  $ btrfs prop get /btrfs compression
    compression=lzo
  $ btrfs prop set /btrfs compression zli
    ERROR: failed to set compression for /btrfs: Invalid argument
  $ btrfs prop get /btrfs compression

This is because the compression property ->validate() is successful for
'zli' as the strncmp() used the length passed from the userspace.

Fix it by using the expected string length in strncmp().

Fixes: 63541927 ("Btrfs: add support for inode properties")
Fixes: 5c1aab1d ("btrfs: Add zstd support")
CC: stable@vger.kernel.org # 4.14+
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/props.c

@@ -366,11 +366,11 @@ int btrfs_subvol_inherit_props(struct btrfs_trans_handle *trans,
 
 static int prop_compression_validate(const char *value, size_t len)
 {
-	if (!strncmp("lzo", value, len))
+	if (!strncmp("lzo", value, 3))
 		return 0;
-	else if (!strncmp("zlib", value, len))
+	else if (!strncmp("zlib", value, 4))
 		return 0;
-	else if (!strncmp("zstd", value, len))
+	else if (!strncmp("zstd", value, 4))
 		return 0;
 
 	return -EINVAL;