With this commit, the LSM Smack implements the LSM
side part of the system call keyctl with the action
code KEYCTL_GET_SECURITY.

It is now possible to get the context of, for example,
the user session key using the command "keyctl security @s".

The original patch has been modified for merge.
Signed-off-by: NJosé Bollo <jose.bollo@open.eurogiciel.org>
Signed-off-by: NCasey Schaufler <casey@schaufler-ca.com>