mm/mmu_notifier.c: remove mmu_notifier_synchronize()

mm/mmu_notifier.c: remove mmu_notifier_synchronize()

Contrary to its name, mmu_notifier_synchronize() does not synchronize the
notifier's SRCU instance, but rather waits for RCU callbacks to finished,
i.e.  it invokes rcu_barrier().  The RCU documentation is quite clear on
this matter, explicitly calling out that rcu_barrier() does not imply
synchronize_rcu().

As there are no callers of mmu_notifier_synchronize() and it's unclear
whether any user of mmu_notifier_call_srcu() will ever want to barrier on
their callbacks, simply remove the function.

Link: http://lkml.kernel.org/r/20181106134705.14197-1-sean.j.christopherson@intel.com
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>