x86/traps: remove smp_mb() ahead of IPI dispatch

The IPI dispatch functions already have the required barriers to ensure
correct memory ordering.

Note other callers of send_IPI_mask() don't use any barriers.

No functional change intended.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/traps.c

@@ -785,8 +785,6 @@ void fatal_trap(const struct cpu_user_regs *regs, bool show_remote)
             cpumask_andnot(&show_state_mask, &cpu_online_map,
                            cpumask_of(smp_processor_id()));
             set_nmi_callback(nmi_show_execution_state);
-            /* Ensure new callback is set before sending out the NMI. */
-            smp_wmb();
             smp_send_nmi_allbutself();
 
             /* Wait at most 10ms for some other CPU to respond. */