selinux: Fix a panic in selinux_netlbl_inode_permission()

Rick McNeal from LSI identified a panic in selinux_netlbl_inode_permission()
caused by a certain sequence of SUNRPC operations.  The problem appears to be
due to the lack of NULL pointer checking in the function; this patch adds the
pointer checks so the function will exit safely in the cases where the socket
is not completely initialized.
Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>

security/selinux/netlabel.c

@@ -386,11 +386,12 @@ int selinux_netlbl_inode_permission(struct inode *inode, int mask)
 	if (!S_ISSOCK(inode->i_mode) ||
 	    ((mask & (MAY_WRITE | MAY_APPEND)) == 0))
 		return 0;
-
 	sock = SOCKET_I(inode);
 	sk = sock->sk;
+	if (sk == NULL)
+		return 0;
 	sksec = sk->sk_security;
-	if (sksec->nlbl_state != NLBL_REQUIRE)
+	if (sksec == NULL || sksec->nlbl_state != NLBL_REQUIRE)
 		return 0;
 
 	local_bh_disable();