x86/extable: Adjust extable handling to be shadow stack compatible

x86/extable: Adjust extable handling to be shadow stack compatible

When adjusting an IRET frame to recover from a fault, and equivalent
adjustment needs making in the shadow IRET frame.

The adjustment in exception_with_ints_disabled() could in principle be an
alternative block rather than an ifdef, as the only two current users of
_PRE_EXTABLE() are IRET-to-guest instructions.  However, this is not a
fastpath, and this form is more robust to future changes.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>