btrfs: hoist fs_info out of loops in end_bbio_data_write and end_bbio_data_read

The fs_info and sectorsize remain the same during the loops, no need to
set them on each iteration.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>