LSM: fix documentation for the shm_* hooks

LSM: fix documentation for the shm_* hooks

The shm_* hooks were changed in the commit
"shm/security: Pass kern_ipc_perm not shmid_kernel into the
shm security hooks" (7191adff2a55). The type of the argument
shp was changed from shmid_kernel to kern_ipc_perm. This patch
updates the documentation for the hooks accordingly.

Signed-off-by: Denis Efremov <efremov@ispras.ru>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: James Morris <james.morris@microsoft.com>