Protect nfs socket locks with a critical section. Recheck rep->r_mrep just

Protect nfs socket locks with a critical section.  Recheck rep->r_mrep just
prior to calling tsleep() in case another thread got in and handled the
request being waited for.  Rewrite the vnode scanning code in nfs_sync()
to use vmntvnodescan(), fixing a number of potential races.  Protect the
commit phase 2 scan in nfs_subs.c with the appropriate token (note: still
needs some work).