KVM: x86/mmu: Lazily allocate memslot rmaps

mainline inclusion
from mainline-v5.14-rc1
commit d501f747
category: feature
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I7S3VQ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d501f747ef5c0ac0c917f9a6781d04ae4ae39d63



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If the TDP MMU is in use, wait to allocate the rmaps until the shadow
MMU is actually used. (i.e. a nested VM is launched.) This saves memory
equal to 0.2% of guest memory in cases where the TDP MMU is used and
there are no nested guests involved.

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210518173414.450044-8-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

conflict:
	arch/x86/include/asm/kvm_host.h

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>