Create MAXBCACHEBUF as a separate constant from MAXBSIZE

[rmacklem]

Summary

MAXBSIZE defines both the largest UFS block size and the largest buffer
cache block size. This patch defines the latter as a separate constant
called MAXBCACHEBUF, so that it can be defined on a per-architecture
basis. This allows it to be made larger than MAXBSIZE, so that NFS
can use larger read/write data sizes.

Test Plan

Already tested by me and mav@.