Make pass(4) handle misaligned buffers of MAXPHYS size.

Since we are already using malloc()+copyin()/copyout() for smaller data
blocks, and since new asynchronous API does it always, I see no reason
to keep this ugly artificial size/alignment limitation in old API.

Tape applications suffer enough from the MAXPHYS limitations by itself,
and additional alignment requirement, often halving effectively usable
block size, does not help.

It would be good to use unmapped I/O here instead, but it require some
HBA drivers polishing first to support non-BIO unmapped buffers.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.