x86: introduce max_pfn_mapped

64bit uses end_pfn_map and 32bit uses max_low_pfn. There are several
files which have #ifdef'ed defines which map either to end_pfn_map or
max_low_pfn. Replace this by a universal define and clean up all the
other instances.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>