btrfs: remove zero length check when entering fiemap

There's no point to check for a 0 length at extent_fiemap(), as before
calling it, we called fiemap_prep() at btrfs_fiemap(), which already
checks for a zero length and returns the same -EINVAL error. So remove
the pointless check.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_io.c

@@ -5455,9 +5455,6 @@ int extent_fiemap(struct btrfs_inode *inode, struct fiemap_extent_info *fieinfo,
 	u64 em_len = 0;
 	u64 em_end = 0;
 
-	if (len == 0)
-		return -EINVAL;
-
 	path = btrfs_alloc_path();
 	if (!path)
 		return -ENOMEM;