x86: fix pte_modify() bug

fix sign extension bug in PTE_MASK / _PTE_CHG_MASK.

this resolves the following bootup crash on PAE systems:

[   94.710726] init[1]: segfault at 00000004 ip 49471cbb sp bff0c6c0 error 4
[   94.717764] init[1]: segfault at 00000004 ip 49471cbb sp bff0c6c0 error 4
[   94.724772] init[1]: segfault at 00000004 ip 49471cbb sp bff0c6c0 error 4
[   94.731777] init[1]: segfault at 00000004 ip 49471cbb sp bff0c6c0 error 4

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>