metag: copy_from_user() should zero the destination on access_ok() failure

[ Upstream commit 8ae95ed4ae5fc7c3391ed668b2014c9e2079533b ]

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