x86/entry: Change exit path of xen_failsafe_callback

xen_failsafe_callback() is invoked from XEN for two cases:

  1. Fault while reloading DS, ES, FS or GS
  2. Fault while executing IRET

 #1 retries the IRET after XEN has fixed up the segments.
 #2 injects a #GP which kills the task

For #1 there is no reason to go through the full exception return path
because the tasks TIF state is still the same. So just going straight to
the IRET path is good enough.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20200521202118.423224507@linutronix.de