aio: Fix opcode handling in aio_process_rw()

LIO_FOFFSET needs to be masked off, as it is in aio_aqueue().

Reported by: syzbot+b6e15476c91852bb2264@syzkaller.appspotmail.com
Reviewed by: kib, asomers
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D50118