target-i386: mmu: fix handling of noncanonical virtual addresses

target-i386: mmu: fix handling of noncanonical virtual addresses

mmu_translate is supposed to return an error code for page faults; it is
not able to handle other exceptions.  The #GP case for noncanonical
virtual addresses is not handled correctly, and incorrectly raised as
a page fault with error code 1.  Since it cannot happen for nested
page tables, move it directly to handle_mmu_fault, even before the
invocation of mmu_translate.

Fixes: #676
Fixes: 661ff4879e ("target/i386: extract mmu_translate", 2021-05-11)
Cc: qemu-stable@nongnu.org
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>