btrfs: replace div_u64 by shift in free_space_bitmap_size

Change free_space_bitmap_size to take btrfs_fs_info so we can get the
sectorsize_bits to do calculations.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>