scsi: core: Make SCSI_MOD depend on BLOCK for cleaner .config files

SCSI_MOD is a helper config symbol for configuring RAID_ATTRS properly,
i.e., RAID_ATTRS needs to be m when SCSI=m.

This helper config symbol SCSI_MOD still shows up even in kernel
configurations that do not select the block subsystem and where SCSI is not
even a configuration option mentioned and selectable.

Make this SCSI_MOD depend on BLOCK, so that it only shows up when it is
slightly relevant in the kernel configuration.

Link: https://lore.kernel.org/r/20220919060112.24802-1-lukas.bulwahn@gmail.com
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>