[NetLabel]: make the CIPSOv4 cache spinlocks bottom half safe

The CIPSOv4 cache traversal routines are triggered both the userspace events
(cache invalidation due to DOI removal or updated SELinux policy) and network
packet processing events.  As a result there is a problem with the existing
CIPSOv4 cache spinlocks as they are not bottom-half/softirq safe.  This patch
converts the CIPSOv4 cache spin_[un]lock() calls into spin_[un]lock_bh() calls
to address this problem.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>