lio_listio(2): add LIO_FOFFSET flag to ignore aiocb aio_offset

and use the current file offset instead.

Requested by: Vinícius dos Santos Oliveira <vini.ipsmaker@gmail.com>
Reviewed by: jhb
Discussed with: asomers
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D43448