SELinux: Made netnode cache adds faster

When adding new entries to the network node cache we would walk the entire
hash bucket to make sure we didn't cross a threshold (done to bound the
cache size).  This isn't a very quick or elegant solution for something
which is supposed to be quick-ish so add a counter to each hash bucket to
track the size of the bucket and eliminate the need to walk the entire
bucket list on each add.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>