KVM: arm64: Omit tag sync on stage-2 mappings of the zero page

KVM: arm64: Omit tag sync on stage-2 mappings of the zero page

Commit

   f620d66af316 ("arm64: mte: Do not flag the zero page as PG_mte_tagged")

removed the PG_mte_tagged flag from the zero page, but missed a KVM code
path that may set this flag on the zero page when it is used in a
stage-2 CoW mapping of anonymous memory.

So disregard the zero page explicitly in sanitise_mte_tags().

Fixes: f620d66af316 ("arm64: mte: Do not flag the zero page as PG_mte_tagged")
Cc: stable@vger.kernel.org # 5.10.x
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>