Introduce virt_to_phys()

virt_to_phys() returns the physical address given a virtual. In most
cases this will be just the input value as the vast majority of
systems run in a 1:1 mode.

However in systems that are not running this way it should report the
physical address or ~0 if no mapping exists for the given virtual
address.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>