aio: micro-optimize the lio_opcode assignments

This allows slightly more efficient opcode testing in-kernel. It is
transparent to userland, except to applications that sneakily submit
aio fsync or aio mlock operations via lio_listio, which has never been
documented, requires the use of deliberately undefined constants
(LIO_SYNC and LIO_MLOCK), and is arguably a bug.

Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D27942