x86: mtrr_cleanup try gran_size to less than 1M, v3

J.A. Magallón reported:

 >> Also, on a 64 bit box with 4Gb, it gives this:
 >>
 >> cicely:~# cat /proc/mtrr
 >> reg00: base=0x00000000 (   0MB), size=4096MB: write-back, count=1
 >> reg01: base=0x100000000 (4096MB), size=1024MB: write-back, count=1
 >> reg02: base=0x140000000 (5120MB), size= 512MB: write-back, count=1
 >> reg03: base=0x160000000 (5632MB), size= 256MB: write-back, count=1
 >> reg04: base=0x80000000 (2048MB), size=2048MB: uncachable, count=1

boundary handling has a problem ... fix it.

Reported-by: J.A. Magallón <jamagallon@ono.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>