LSM: fix documentation for the socket_getpeersec_dgram hook

The socket_getpeersec_dgram hook was changed in the commit
"[AF_UNIX]: Kernel memory leak fix for af_unix datagram
getpeersec patch" (dc49c1f9). The arguments @secdata
and @seclen were changed to @sock and @secid. This patch
updates the documentation 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>