lio_listio: validate aio_lio_opcode

Previously, we would accept any kind of LIO_* opcode, including ones
that were intended for in-kernel use only like LIO_SYNC (which is not
defined in userland). The situation became more serious with
022ca2fc7fe08d51f33a1d23a9be49e6d132914e. After that revision, setting
aio_lio_opcode to LIO_WRITEV or LIO_READV would trigger an assertion.

Note that POSIX does not specify what should happen if aio_lio_opcode is
invalid.

MFC-with: 022ca2fc7fe08d51f33a1d23a9be49e6d132914e
Reviewed by: jhb, tmunro, 0mp
Differential Revision: <https://reviews.freebsd.org/D28078