security: Add a hook for the point of notification insertion

Add a security hook that allows an LSM to rule on whether a notification
message is allowed to be inserted into a particular watch queue.

The hook is given the following information:

 (1) The credentials of the triggerer (which may be init_cred for a system
     notification, eg. a hardware error).

 (2) The credentials of the whoever set the watch.

 (3) The notification message.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: James Morris <jamorris@linux.microsoft.com>
cc: Casey Schaufler <casey@schaufler-ca.com>
cc: Stephen Smalley <sds@tycho.nsa.gov>
cc: linux-security-module@vger.kernel.org