KVM: x86/mmu: Alloc TDP MMU roots while holding mmu_lock for read

Allocate TDP MMU roots while holding mmu_lock for read, and instead use
tdp_mmu_pages_lock to guard against duplicate roots.  This allows KVM to
create new roots without forcing kvm_tdp_mmu_zap_invalidated_roots() to
yield, e.g. allows vCPUs to load new roots after memslot deletion without
forcing the zap thread to detect contention and yield (or complete if the
kernel isn't preemptible).

Note, creating a new TDP MMU root as an mmu_lock reader is safe for two
reasons: (1) paths that must guarantee all roots/SPTEs are *visited* take
mmu_lock for write and so are still mutually exclusive, e.g. mmu_notifier
invalidations, and (2) paths that require all roots/SPTEs to *observe*
some given state without holding mmu_lock for write must ensure freshness
through some other means, e.g. toggling dirty logging must first wait for
SRCU readers to recognize the memslot flags change before processing
existing roots/SPTEs.

Link: https://lore.kernel.org/r/20240111020048.844847-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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