With lio_listio(2), the opcode is specified by userspace rather than
being hard-coded by the system call (e.g., aio_readv() -> LIO_READV).
kern_lio_listio() calls aio_aqueue() with an opcode of LIO_NOP, which
gets fixed up when the aiocb is copied in.

When copying in a job request, we need to dynamically allocate a uio to
wrap the iovec. So aiocb_copyin() needs to get the opcode from the
aiocb and then decide whether an allocation is required. We failed to
do this in the COMPAT_FREEBSD32 case. Fix it.

Reported by: syzbot+27eab6f2c2162f2885ee@syzkaller.appspotmail.com
Fixes: f30a1ae8d529 ("lio_listio(2): Allow LIO_READV and LIO_WRITEV.")