KVM: mmu: Add slots_arch_lock for memslot arch fields

mainline inclusion
from mainline-v5.14-rc1
commit b10a038e
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=b10a038e84d188e15819058b2978b2daa9853aeb



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Add a new lock to protect the arch-specific fields of memslots if they
need to be modified in a kvm->srcu read critical section. A future
commit will use this lock to lazily allocate memslot rmaps for x86.

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210518173414.450044-5-bgardon@google.com>
[Add Documentation/ hunk. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

conflictt:
	virt/kvm/kvm_main.c

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