x86: Squash outstanding walks when instructions are squashed.

This is the x86 version of the ARM changeset 9258 (http://repo.gem5.org/gem5/rev/baa17ba80e06) and the related discussions at http://www.mail-archive.com/gem5-users@gem5.org/msg02810.html

Tested with gem5 tip using x86_64-vmlinux-2.6.22.9 and a custom application that trips the assertion error : cpu->instcount <= 1500. When 4 translations are squashed per cycle, the assertion is no longer tripped. (With 2 squashes per cycle, the assertion is still hit.)