NFSD: Remove NFSSVC_MAXBLKSIZE_V2 macro

The 8192-byte maximum is a protocol-defined limit, and we already
have a symbolic constant defined whose name matches the name of
the limit defined in the protocol. Replace the duplicate.

No change in behavior is expected.

Reviewed-by: NeilBrown <neil@brown.name>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>