s390/futex: add missing EX_TABLE entry to __futex_atomic_op()

commit a262d3ad6a433e4080cecd0a8841104a5906355e upstream.

For some exception types the instruction address points behind the
instruction that caused the exception. Take that into account and add
the missing exception table entry.

Cc: <stable@vger.kernel.org>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>