xen/p2m: m2p_find_override: use list_for_each_entry_safe

Use list_for_each_entry_safe and remove the spin_lock acquisition in
m2p_find_override.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>