x86: fix trim mtrr not to setup_memory two times

we could call find_max_pfn() directly instead of setup_memory() to get
max_pfn needed for mtrr trimming.

otherwise setup_memory() is called two times... that is duplicated...

[ mingo@elte.hu: both Thomas and me simulated a double call to
  setup_bootmem_allocator() and can confirm that it is a real bug
  which can hang in certain configs. It's not been reported yet but
  that is probably due to the relatively scarce nature of
  MTRR-trimming systems. ]

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>