selinux: reduce the number of calls to synchronize_net() when flushing caches

commit 615e51fd upstream.

When flushing the AVC, such as during a policy load, the various
network caches are also flushed, with each making a call to
synchronize_net() which has shown to be expensive in some cases.
This patch consolidates the network cache flushes into a single AVC
callback which only calls synchronize_net() once for each AVC cache
flush.

Change-Id: I2a7f020748d1adf2b68246f6ef86d0c871adffb7
Reported-by: Jaejyn Shin <flagon22bass@gmail.com>
Signed-off-by: Paul Moore <pmoore@redhat.com>