patch that makes max buffer cache block size tunable for review

[Rick Macklem]

Hi,

I just put a patch here:
https://reviews.freebsd.org/D10991
that makes the maximum size of a buffer cache block a tunable.

This allows the NFS client to use larger I/O sized RPCs.
By default, the NFS client will use the largest I/O size possible.
What is actually in use can be checked via "nfsstat -m".

Anyone interested in reviewing and/or testing this, please do so, rick