btrfs: stop checking for NULL return from btrfs_get_extent_fiemap()

In get_extent_skip_holes() we're checking the return of
btrfs_get_extent_fiemap() for an error pointer or NULL, but
btrfs_get_extent_fiemap() will never return NULL, only error pointers or
a valid extent_map.

The other caller of btrfs_get_extent_fiemap(), find_desired_extent(),
correctly only checks for error-pointers.
Reviewed-by: NFilipe Manana <fdmanana@suse.com>
Signed-off-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -5390,7 +5390,7 @@ static struct extent_map *get_extent_skip_holes(struct btrfs_inode *inode,
 			break;
 		len = ALIGN(len, sectorsize);
 		em = btrfs_get_extent_fiemap(inode, offset, len);
-		if (IS_ERR_OR_NULL(em))
+		if (IS_ERR(em))
 			return em;
 
 		/* if this isn't a hole return it */