NetLabel: correct locking in selinux_netlbl_socket_setsid()

The spinlock protecting the update of the "sksec->nlbl_state" variable is not
currently softirq safe which can lead to problems.  This patch fixes this by
changing the spin_{un}lock() functions into spin_{un}lock_bh() functions.

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