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: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>