generic: make PFN_PHYS explicitly return phys_addr_t

PFN_PHYS, as its name suggests, turns a pfn into a physical address.
However, it is a macro which just operates on its argument without
modifying its type.  pfns are typed unsigned long, but an unsigned
long may not be long enough to hold a physical address (32-bit systems
with more than 32 bits of physcial address).

Make sure we cast to phys_addr_t to return a complete result.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>