[NetLabel]: add missing rcu_dereference() calls in the LSM domain mapping hash table

The LSM domain mapping head table pointer was not being referenced via the RCU
safe dereferencing function, rcu_dereference().  This patch adds those missing
calls to the NetLabel code.

This has been tested using recent linux-2.6 git kernels with no visible
regressions.

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