KVM: x86/mmu: Honor NEED_RESCHED when zapping rmaps and blocking is allowed

Convert kvm_unmap_gfn_range(), which is the helper that zaps rmap SPTEs in
response to an mmu_notifier invalidation, to use __kvm_rmap_zap_gfn_range()
and feed in range->may_block.  In other words, honor NEED_RESCHED by way of
cond_resched() when zapping rmaps.  This fixes a long-standing issue where
KVM could process an absurd number of rmap entries without ever yielding,
e.g. if an mmu_notifier fired on a PUD (or larger) range.

Opportunistically rename __kvm_zap_rmap() to kvm_zap_rmap(), and drop the
old kvm_zap_rmap().  Ideally, the shuffling would be done in a different
patch, but that just makes the compiler unhappy, e.g.

  arch/x86/kvm/mmu/mmu.c:1462:13: error: ‘kvm_zap_rmap’ defined but not used

Reported-by: Peter Xu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

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