x86/entry: Fix instrumentation annotation

Embracing a callout into instrumentation_begin() / instrumentation_begin()
does not really make sense. Make the latter instrumentation_end().

Fixes: 2f6474e4 ("x86/entry: Switch XEN/PV hypercall entry to IDTENTRY")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20210210002512.106502464@linutronix.de