LSM: fix documentation for the path_chmod hook

The path_chmod hook was changed in the commit
"switch security_path_chmod() to struct path *" (cdcf116d

).
The argument @mnt was removed from the hook, @dentry was changed
to @path. 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>