nfsd: Make modifying vfs.nfsd.enable_locallocks safe

Commit dfaeeacc2cc2 modified clientID handling so that it could be done
with only a mutex lock held when vfs.nfsd.enable_locallocks is 0.
This makes it unsafe to change the setting of vfs.nfsd.enable_locallocks
when nfsd threads are active.

This patch forces all nfsd threads to be blocked when the value
of vfs.nfsd.enable_locallocks is changed, so that it is done safely.

MFC after: 1 month