x86, mm: limit MAXMEM on 64-bit

on 64-bit x86 the physical memory limit is controlled by the sparsemem
bits - which are 44 bits right now. But MAXMEM (the max pfn number
e820 parsing will allow to enter our sizing routines) is set to
0x00003fffffffffff, i.e. 46 bits - that's too large because it overlaps
into the vmalloc range.

So couple MAXMEM to MAX_PHYSMEM_BITS, and add a comment that the
maximum of MAX_PHYSMEM_BITS is 45 bits.

Signed-off-by: Ingo Molnar <mingo@elte.hu>