KVM: MMU: fast page fault

Changlog:
- always atomicly update the spte if it can be updated out of mmu-lock
- rename spte_can_be_writable() to spte_is_locklessly_modifiable()
- cleanup and comment spte_write_protect()

Performance result:
(The benchmark can be found at: http://www.spinics.net/lists/kvm/msg73011.html)

                               before            after
Run 10 times, Avg time:      538233957 ns.      249809853 ns. +53.6%

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