[SCSI] scsi_dh: cosmetic change to sizeof()

instead of doing sizeof(struct X) it's better to do sizeof(*v) where v
is the variable pointing to struct X.

Signed-off-by: Hillf Danton <dhillf@gmail.com>
Signed-off-by: Babu Moger <babu.moger@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>