x86: AMD Athlon X2 hard hang fix

An Athlon 64 X2 test system showed hard hangs shortly after marking
the kernel text read-only, if we tried to preserve largepages and
changed the PSE entry from RW to RO. The pagetable code itself is
correct, it's the CPU that locked up hard (and not even the NMI
watchdog could punch through that hard hang).

So be conservative and always do splitups - like we did in the past.

Signed-off-by: Ingo Molnar <mingo@elte.hu>