Fix the fact that pte_mkread set _PAGE_RW instead of _PAGE_USER (the logic is
copied from i386 in most place, so it is really as bad as you're thinking).

Thus currently page tables are more permissive than they should.

Such a change may trigger other latent bugs, so be careful with this.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>