NetLabel: Consolidate the LSM domain mapping/hashing locks

Currently we use two separate spinlocks to protect both the hash/mapping table
and the default entry.  This could be considered a bit foolish because it adds
complexity without offering any real performance advantage.  This patch
removes the dedicated default spinlock and protects the default entry with the
hash/mapping table spinlock.

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