selinux: properly handle multiple messages in selinux_netlink_send()

commit fb739741 upstream.

Fix the SELinux netlink_send hook to properly handle multiple netlink
messages in a single sk_buff; each message is parsed and subject to
SELinux access control.  Prior to this patch, SELinux only inspected
the first message in the sk_buff.

Bug: 155485360
Cc: stable@vger.kernel.org
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I903740dddb0289d5441a961599242e87393268c0