lsm: introduce an initcall mechanism into the LSM framework

lsm: introduce an initcall mechanism into the LSM framework

Currently the individual LSMs register their own initcalls, and while
this should be harmless, it can be wasteful in the case where a LSM
is disabled at boot as the initcall will still be executed.  This
patch introduces support for managing the initcalls in the LSM
framework, and future patches will convert the existing LSMs over to
this new mechanism.

Only initcall types which are used by the current in-tree LSMs are
supported, additional initcall types can easily be added in the future
if needed.

Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: John Johansen <john.johhansen@canonical.com>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>