LSM: fix documentation for the socket_post_create hook

This patch slightly fixes the documentation for the
socket_post_create hook. The documentation states that
i_security field is accessible through inode field of socket
structure (i.e., 'sock->inode->i_security'). There is no inode
field in the socket structure. The i_security field is accessible
through SOCK_INODE macro. The patch updates the documentation
to reflect this.

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>