target-i386: Don't forbid NX bit on PAE PDEs and PTEs

Commit e8f6d00c ("target-i386: raise
page fault for reserved physical address bits") added a check that the
NX bit is not set on PAE PDPEs, but it also added it to rsvd_mask for
the rest of the function. This caused any PDEs or PTEs with NX set to be
erroneously rejected, making PAE guests with NX support unusable.
Signed-off-by: NWilliam Grant <wgrant@ubuntu.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 1844e68e)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

target-i386/helper.c

@@ -615,8 +615,8 @@ int x86_cpu_handle_mmu_fault(CPUState *cs, vaddr addr,
             if (!(pdpe & PG_PRESENT_MASK)) {
                 goto do_fault;
             }
-            rsvd_mask |= PG_HI_USER_MASK | PG_NX_MASK;
-            if (pdpe & rsvd_mask) {
+            rsvd_mask |= PG_HI_USER_MASK;
+            if (pdpe & (rsvd_mask | PG_NX_MASK)) {
                 goto do_fault_rsvd;
             }
             ptep = PG_NX_MASK | PG_USER_MASK | PG_RW_MASK;