scsi: Suppress gcc 7 fall-through warnings reported with W=1

The conclusion of a recent discussion about the new warnings
reported by gcc 7 is that the new warnings reported when building
with W=1 should be suppressed. However, gcc 7 still warns about
fall-through in switch statements when building with W=1. Suppress
these warnings by annotating the SCSI core properly.

See also Linus Torvalds, Lots of new warnings with gcc-7.1.1, 11
July 2017 (https://www.mail-archive.com/linux-media@vger.kernel.org/msg115428.html

).

References: commit bd664f6b ("disable new gcc-7.1.1 warnings for now")
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>