nfsd: Add a new rc variable nfs_server_maxio

Since vfs.nfsd.srvmaxio can only be set when nfsd.ko
is loaded, but nfsd is not running, setting it in
/etc/sysctl.conf is not feasible when "options NFSD"
was not specified for the kernel.

This patch adds a new rc variable nfs_server_maxio,
which sets vfs.nfsd.srvmaxio at the correct time.

rc.conf.5 will be patched separately.

Reviewed by: 0mp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D32997