btrfs: remove the unused parameter @len for btrfs_bio_fits_in_stripe()

The parameter @len is not really used in btrfs_bio_fits_in_stripe(),
just remove it.

It got removed in 42034313 ("btrfs: let callers of
btrfs_get_io_geometry pass the em"), before that btrfs_get_chunk_map
utilized it.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>