x86-64/smp: fix random SIGSEGV issues

They seem to have been due to AMD errata 63/122; the fix is to disable
TLB flush filtering in SMP configurations.

Confirmed to fix the problem by Andrew Walrond <andrew@walrond.org>

[ Let's see if we'll have a better fix eventually, this is the Q&D
  "let's get this fixed and out there" version ]

Signed-off-by: Linus Torvalds <torvalds@osdl.org>