KVM: MMU: Atomically check for accessed bit when dropping an spte

Currently, in the window between the check for the accessed bit, and actually
dropping the spte, a vcpu can access the page through the spte and set the bit,
which will be ignored by the mmu.

Fix by using an exchange operation to atmoically fetch the spte and drop it.

Signed-off-by: Avi Kivity <avi@redhat.com>