scsi_wait_scan was introduced with asynchronous host scanning as a hack
for distributions that weren't using proper udev based wait for root to
appear in their initramfs scripts.  In 2.6.30 Commit

c7510859


Author: Rafael J. Wysocki <rjw@sisk.pl>
Date:   Sun Apr 12 20:06:56 2009 +0200

    PM/Hibernate: Wait for SCSI devices scan to complete during resume

Actually broke scsi_wait_scan because it renders
scsi_complete_async_scans() a nop for modular SCSI if you include
scsi_scans.h (which this module does).

The lack of bug reports is sufficient proof that this module is no
longer used.

Cc: Jeff Mahoney <jeffm@suse.de>
Cc: Dave Jones <davej@redhat.com>
Cc: maximilian attems <max@stro.at>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>