KVM: arm64: Use kvm_s2_fault_vma_info in gmem_abort()

gmem_abort() maps at s2fd->fault_ipa, which HPFAR_EL2 holds at 4K
granularity whatever the page size. The generic page-table code aligns
it, but pkvm_pgtable_stage2_map() looks up existing mappings over
[addr, addr + size), so on a pKVM host with pages larger than 4K a
guest_memfd-backed guest that faults past the first 4K of a page can
find its neighbour's mapping, get -EAGAIN and take the same fault
forever.

Take the addresses from kvm_s2_fault_vma_info instead, as
user_mem_abort() does, and report the gfn kvm_gmem_get_pfn() failed on
in the memory fault exit. kvm_s2_fault_get_vma_info() itself isn't
called: a guest_memfd memslot's userspace_addr doesn't need to be
backed by a VMA.

Fixes: a7b57e0995927 ("KVM: arm64: Handle guest_memfd-backed guest page faults")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/r/20260913175105.A57AC1F000FF@smtp.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>