[SCSI] st: Store page order before driver buffer allocation

The order of the pages allocated for the driver buffer must be stored before
allocation because it is used in freeing already allocated pages if
allocation fails.

Signed-off-by: Kai Makisara <kai.makisara@kolumbus.fi>
Reported-by: Lukas Kolbe <lkolbe@techfak.uni-bielefeld.de>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>