[PATCH] ppc64 POWER3 segment table fix

[PATCH] ppc64 POWER3 segment table fix

The ppc64 fix last week (enforcing permissions on the kernel when
accessing userspace pages) uncovered a bug on POWER3/RS64. We werent
zeroing the segment table entry before overwriting it and it was possible
for the ks bit to be set on a kernel segment.

The VSID mask was also changed to match reality (we only use 13 bits).