NFSD: Remove NFSD_BUFSIZE

Clean up: The documenting comment for NFSD_BUFSIZE is quite stale.
NFSD_BUFSIZE is used only for NFSv4 Reply these days; never for
NFSv2 or v3, and never for RPC Calls. Even so, the byte count
estimate does not include the size of the NFSv4 COMPOUND Reply
HEADER or the RPC auth flavor.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>