scsi: core: Rename CONFIG_BLK_SCSI_REQUEST to CONFIG_SCSI_COMMON

CONFIG_BLK_SCSI_REQUEST is rather misnamed as it enables building a small
amount of code shared by the SCSI initiator, target, and consumers of the
scsi_request passthrough API.  Rename it and also allow building it as a
module.

[mkp: add module license]

Link: https://lore.kernel.org/r/20210724072033.1284840-20-hch@lst.de


Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>