metag: fix put_user sparse errors

virtio wants to write bitwise types to userspace using put_user.
At the moment this triggers sparse errors, since the value is passed
through an integer.

For example:

	__le32 __user *p;
	__le32 x;
	put_user(x, p);

is safe, but currently triggers a sparse warning.

Fix that up using __force.

This also fixes warnings due to writing a pointer out to
userland.

Note: this does not suppress any useful sparse checks since callers
do a cast (__typeof__(*(ptr))) (x) which in turn forces all the
necessary type checks.

Suggested-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: James Hogan <james.hogan@imgtec.com>