In r264504, we prevented doing I/O for more than MAXPHYS by making
the assumption that consumers would respect bio_completed and/or
bio_resid to detect short reads. This assumption proved false and
file corruption was the result.
Create as many bios as we need to satisfy the original request.
Check the cached chunk every time we need to do I/O to increase the
hit rate.

Obtained from:	junipre Networks, Inc.
MFC after:	1 week