block: add blk_rq_set_block_pc()

With the optimizations around not clearing the full request at alloc
time, we are leaving some of the needed init for REQ_TYPE_BLOCK_PC
up to the user allocating the request.

Add a blk_rq_set_block_pc() that sets the command type to
REQ_TYPE_BLOCK_PC, and properly initializes the members associated
with this type of request. Update callers to use this function instead
of manipulating rq->cmd_type directly.

Includes fixes from Christoph Hellwig <hch@lst.de> for my half-assed
attempt.

Signed-off-by: Jens Axboe <axboe@fb.com>

diff --cc block/blk-core.c
Simple merge

diff --cc block/bsg.c
Simple merge

diff --cc block/scsi_ioctl.c
Simple merge

diff --cc drivers/block/pktcdvd.c
Simple merge

diff --cc drivers/cdrom/cdrom.c
Simple merge

diff --cc drivers/scsi/device_handler/scsi_dh_alua.c
Simple merge

diff --cc drivers/scsi/device_handler/scsi_dh_emc.c
Simple merge

diff --cc drivers/scsi/device_handler/scsi_dh_hp_sw.c
Simple merge

diff --cc drivers/scsi/device_handler/scsi_dh_rdac.c
Simple merge

diff --cc drivers/scsi/osd/osd_initiator.c
Simple merge

diff --cc drivers/scsi/osst.c
Simple merge

diff --cc drivers/scsi/scsi_error.c
Simple merge

diff --cc drivers/scsi/scsi_lib.c
Simple merge

diff --cc drivers/scsi/sg.c
Simple merge

diff --cc drivers/scsi/st.c
Simple merge

diff --cc drivers/target/target_core_pscsi.c
Simple merge

diff --cc include/linux/blkdev.h
Simple merge