target/i386: leave the A20 bit set in the final NPT walk

target/i386: leave the A20 bit set in the final NPT walk

The A20 mask is only applied to the final memory access.  Nested
page tables are always walked with the raw guest-physical address.

Unlike the previous patch, in this one the masking must be kept, but
it was done too early.

Cc: qemu-stable@nongnu.org
Fixes: 4a1e9d4d11c ("target/i386: Use atomic operations for pte updates", 2022-10-18)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit b5a9de3259f4c791bde2faff086dd5737625e41e)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>