sunrpc: Add a helper to derive maxpages from sv_max_mesg

This page count is to be used to allocate various arrays of pages
and bio_vecs, replacing the fixed RPCSVC_MAXPAGES value.

The documenting comment is somewhat stale -- of course NFSv4
COMPOUND procedures may have multiple payloads.

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