Revert "xen/grant-table: Avoid m2p_override during mapping"

This reverts commit 08ece5bb.

As it breaks ARM builds and needs more attention
on the ARM side.

Acked-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>