MFC: r299201
Give mountd -S priority over outstanding RPC requests when suspending the nfsd.

It was reported via email that under certain heavy RPC loads
long delays before the exports would be updated was observed
when using "mountd -S". This patch reverses the priority between
the exclusive lock request to suspend the nfsd threads and the
shared lock request for performing RPCs.
As such, when mountd attempts to suspend the nfsd threads, it
gets priority over outstanding RPC requests to do this.
I suspect that the case reported was an artificial test load,
but this patch did fix the problem for the reporter.