bsg: deprecate BIDI support in bsg

Besides the OSD command set that never got traction, the only SCSI
command using bidirectional buffers is XDWRITEREAD in the 10 and 32 byte
variants, which is extremely esoteric and has been removed from the spec
again as of SBC4r15.  It probably doesn't make sense to keep the support
code around just for that, so start deprecating the support.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>