SCSI: fix transfer direction in scsi_lib and st

scsi_prep_fn and st_init_command could issue WRITE requests with zero
buffer length. This may lead to kernel panic or oops with some SCSI
low-level drivers.

Derived from -rc patches from Jens Axboe and James Bottomley.

Patch is reassembled for -stable from patches:
[SCSI] fix panic when ejecting ieee1394 ipod
[SCSI] Consolidate REQ_BLOCK_PC handling path (fix ipod panic)

Depends on patch "SCSI: fix transfer direction in sd (kernel panic when
ejecting iPod)". Also modifies the already correct sr_init_command to
fully match the corresponding -rc patch.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>