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 <[email protected]>
Acked-by: James Morris <[email protected]>
cc: Casey Schaufler <[email protected]>
cc: Stephen Smalley <[email protected]>
cc: [email protected]

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