Revert "x86 PAT: remove CPA WARN_ON for zero pte"

Revert "x86 PAT: remove CPA WARN_ON for zero pte"

This reverts commit 58dab916dfb57328d50deb0aa9b3fc92efa248ff, which
makes my Nehalem come to a nasty crawling almost-halt.  It looks like it
turns off caching of regular kernel RAM, with the understandable
slowdown of a few orders of magnitude as a result.

Acked-by: Ingo Molnar <[email protected]>
Cc: Yinghai Lu <[email protected]>
Cc: Peter Anvin <[email protected]>
Cc: Venkatesh Pallipadi <[email protected]>
Cc: Suresh Siddha <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>