btrfs: Remove redundant assignment in btrfs_get_extent_fiemap

hole_len is only used if the hole falls within the requested range. Make
that explicitly clear by only assigning in the corresponding branch.
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -7005,7 +7005,7 @@ struct extent_map *btrfs_get_extent_fiemap(struct btrfs_inode *inode,
 
 	if (delalloc_len > 0) {
 		u64 hole_start;
-		u64 hole_len = len;
+		u64 hole_len;
 		const u64 hole_end = extent_map_end(hole_em);
 
 		em = alloc_extent_map();