KVM: MMU: Use cmpxchg for pte updates on walk_addr()

In preparation for multi-threaded guest pte walking, use cmpxchg()
when updating guest pte's. This guarantees that the assignment of the
dirty bit can't be lost if two CPU's are faulting the same address
simultaneously.

[avi: fix kunmap_atomic() parameters]

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>