frv: fix clear_user()

[ Upstream commit 3b8767a8f00cc6538ba6b1cf0f88502e2fd2eb90 ]

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

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>