btrfs: fix return value mixup in btrfs_get_extent

btrfs_get_extent() sets variable ret, but out: error path expect error
to be in variable err so the error code is lost.

Fixes: 6bf9e4bd ("btrfs: inode: Verify inode mode to avoid NULL pointer dereference")
CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Pavel Machek (CIP) <pavel@denx.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -6627,7 +6627,7 @@ struct extent_map *btrfs_get_extent(struct btrfs_inode *inode,
 	    extent_type == BTRFS_FILE_EXTENT_PREALLOC) {
 		/* Only regular file could have regular/prealloc extent */
 		if (!S_ISREG(inode->vfs_inode.i_mode)) {
-			ret = -EUCLEAN;
+			err = -EUCLEAN;
 			btrfs_crit(fs_info,
 		"regular/prealloc extent found for non-regular inode %llu",
 				   btrfs_ino(inode));