[S390] dcssblk: prevent early access without own make_request function

When loading a dcss segment with the dcssblk driver, sometimes the
following kind of message appears:

bio too big device dcssblk0 (8 > 0)
Buffer I/O error on device dcssblk0, logical block 172016
..

The fix is to move the disk registration after setting the
make_request function, to avoid calls into generic_make_request
for dcssblock without having the make_request function set up
properly.

Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>