s390: initialize backchain for ext_int_handler()

To allow correct stack backtraces the backchain for the external
interrupt handler is now initialized with zero like it is already
done for example by io_int_handler().
Signed-off-by: NMichael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/entry64.S

@@ -622,6 +622,7 @@ ext_skip:
 	mvc	__PT_R8(64,%r11),__LC_SAVE_AREA_ASYNC
 	stmg	%r8,%r9,__PT_PSW(%r11)
 	TRACE_IRQS_OFF
+	xc	__SF_BACKCHAIN(8,%r15),__SF_BACKCHAIN(%r15)
 	lghi	%r1,4096
 	lgr	%r2,%r11		# pass pointer to pt_regs
 	llgf	%r3,__LC_EXT_CPU_ADDR	# get cpu address + interruption code