KVM: x86/mmu: Remove FNAME(is_self_change_mapping)

Drop FNAME(is_self_change_mapping) and instead rely on
kvm_mmu_hugepage_adjust() to adjust the hugepage accordingly.  Prior to
commit 4cd071d13c5c ("KVM: x86/mmu: Move calls to thp_adjust() down a
level"), the hugepage adjustment was done before allocating new shadow
pages, i.e. failed to restrict the hugepage sizes if a new shadow page
resulted in account_shadowed() changing the disallowed hugepage tracking.

Removing FNAME(is_self_change_mapping) fixes a bug reported by Huang Hang
where KVM unnecessarily forces a 4KiB page.  FNAME(is_self_change_mapping)
has a defect in that it blindly disables _all_ hugepage mappings rather
than trying to reduce the size of the hugepage.  If the guest is writing
to a 1GiB page and the 1GiB is self-referential but a 2MiB page is not,
then KVM can and should create a 2MiB mapping.

Add a comment above the call to kvm_mmu_hugepage_adjust() to call out the
new dependency on adjusting the hugepage size after walking indirect PTEs.

Reported-by: Huang Hang <hhuang@linux.alibaba.com>
Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Link: https://lore.kernel.org/r/20221213125538.81209-1-jiangshanlai@gmail.com
[sean: rework changelog after separating out the emulator change]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20230202182817.407394-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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