mm: don't use _{g,m}fn for defining INVALID_{G,M}FN

mm: don't use _{g,m}fn for defining INVALID_{G,M}FN

INVALID_{G,M}FN are defined using static inline helpers _{g,m}fn.
This means, they cannot be used to initialize a build time static variable:

In file included from mm.c:24:0:
xen/xen/include/xen/mm.h:59:26: error: initializer element is not constant
 #define INVALID_MFN      _mfn(~0UL)

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Tim Deegan <tim@xen.org>
[jb: add parentheses and const]
Acked-by: Jan Beulich <jbeulich@suse.com>