target/i386: Fix CR2 handling for non-canonical addresses

Commit 3563362ddf ("target/i386: Introduce structures for mmu_translate")
accidentally modified CR2 for non-canonical address exceptions while these
should lead to a #GP / #SS instead -- without changing CR2.

Fix that.

A KUT test for this was submitted as [1].

[1] https://lore.kernel.org/kvm/20250612141637.131314-1-minipli@grsecurity.net/

Fixes: 3563362ddf ("target/i386: Introduce structures for mmu_translate")
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Link: https://lore.kernel.org/r/20250612142155.132175-1-minipli@grsecurity.net
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/tcg/system/excp_helper.c

@@ -592,7 +592,8 @@ static bool get_physical_address(CPUX86State *env, vaddr addr,
                 if (sext != 0 && sext != -1) {
                     *err = (TranslateFault){
                         .exception_index = EXCP0D_GPF,
-                        .cr2 = addr,
+                        /* non-canonical #GP doesn't change CR2 */
+                        .cr2 = env->cr[2],
                     };
                     return false;
                 }