x86/xen: Fix noinstr fail in exc_xen_unknown_trap()

Fix:

  vmlinux.o: warning: objtool: exc_xen_unknown_trap()+0x7: call to printk() leaves .noinstr.text section

Fixes: 2e924936 ("x86/xen: avoid warning in Xen pv guest with CONFIG_AMD_MEM_ENCRYPT enabled")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210621120120.606560778@infradead.org

arch/x86/xen/enlighten_pv.c

@@ -592,8 +592,10 @@ DEFINE_IDTENTRY_RAW(xenpv_exc_debug)
 DEFINE_IDTENTRY_RAW(exc_xen_unknown_trap)
 {
 	/* This should never happen and there is no way to handle it. */
+	instrumentation_begin();
 	pr_err("Unknown trap in Xen PV mode.");
 	BUG();
+	instrumentation_end();
 }
 
 #ifdef CONFIG_X86_MCE