Handle NX correctly in pageattr

Second attempt to try to fix an long standing NX bug in change_page_attr.
Hopefully this version will be better.

The kernel text mapping aliases part of the physical memory and needs its
caching attributes changed too.  But when doing this one has to be very
careful to handle the NX bit correctly.

A previous slightly different version of this broke the X server on some RH
systems.  I hope this version will be better, but if you have X server
problems please revert this one.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>