TOMOYO: Fix oops in tomoyo_mount_acl().

TOMOYO: Fix oops in tomoyo_mount_acl().

commit 4e78c724d47e2342aa8fde61f6b8536f662f795f upstream.

In tomoyo_mount_acl() since 2.6.36, kern_path() was called without checking
dev_name != NULL. As a result, an unprivileged user can trigger oops by issuing
mount(NULL, "/", "ext3", 0, NULL) request.
Fix this by checking dev_name != NULL before calling kern_path(dev_name).

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <jmorris@namei.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>