s390/bp: add missing BPENTER to program check handler

When leaving interpretive execution because of a program check BPENTER
should be called like it is done on interrupt exit as well.

Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>