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>