scsi: Get rid of struct fc_bsg_buffer

struct fc_bsg_buffer is just a clone of struct bsg_buffer from bsg-lib,
so use this one instead.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>