KVM: x86/mmu: Make walk_shadow_page_lockless_{begin,end} interoperate with the TDP MMU

mainline inclusion
from mainline-v5.15-rc1
commit c5c8c7c5
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=c5c8c7c53004cb70715320018c3b4287071c1cfd



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Acquire the RCU read lock in walk_shadow_page_lockless_begin and release
it in walk_shadow_page_lockless_end when the TDP MMU is enabled.  This
should not introduce any functional changes but is used in the following
commit to make fast_page_fault interoperate with the TDP MMU.

Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20210713220957.3493520-4-dmatlack@google.com>
[Use if...else instead of if(){return;}]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>