frv: fix clear_user()

commit 3b8767a8f00cc6538ba6b1cf0f88502e2fd2eb90 upstream.

It should check access_ok().  Otherwise a bunch of places turn into
trivially exploitable rootholes.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Willy Tarreau <w@1wt.eu>