[S390] dcss: fix build bug.

Fix this compile bug:

  CC      drivers/s390/block/dcssblk.o
drivers/s390/block/dcssblk.c: In function 'dcssblk_add_store':
drivers/s390/block/dcssblk.c:387: error: implicit declaration of function 'dcssblk_get_segment_by_name'
drivers/s390/block/dcssblk.c:389: error: label 'release_gd' used but not defined
make[1]: *** [drivers/s390/block/dcssblk.o] Error 1
make: *** [drivers/s390/block/] Error 2

Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>