x86-64: fix updating of UREGS_rip when converting sysenter to #GP

(I spotted this copy-and-paste mistake only when backporting c/s
25200:80f4113be500 to 4.1 and 4.0.)

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>

xen/arch/x86/x86_64/entry.S

@@ -280,7 +280,7 @@ sysenter_eflags_saved:
 UNLIKELY_START(z, sysenter_gpf)
         movq  VCPU_trap_ctxt(%rbx),%rsi
         movl  $TRAP_gp_fault,UREGS_entry_vector(%rsp)
-        subl  $2,UREGS_rip(%rsp)
+        subq  $2,UREGS_rip(%rsp)
         movl  %eax,TRAPBOUNCE_error_code(%rdx)
         movq  TRAP_gp_fault * TRAPINFO_sizeof + TRAPINFO_eip(%rsi),%rax
         testb $4,TRAP_gp_fault * TRAPINFO_sizeof + TRAPINFO_flags(%rsi)