NetLabel: Remove unneeded RCU read locks

This patch removes some unneeded RCU read locks as we can treat the reads as
"safe" even without RCU.  It also converts the NetLabel configuration refcount
from a spinlock protected u32 into atomic_t to be more consistent with the rest
of the kernel.

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