cam: don't send scsi commands on shutdown when reboot method RB_NOSYNC
Don't send the SCSI comand SYNCHRONIZE CACHE on devices that are still
open when RB_NOSYNC is the reboot method. This may avoid recursive panics
when doadump is called due to a SCSI/CAM/USB error/bug.
Obtained from: Semihalf
Sponsored by: Stormshield
Reviewed by: imp
Differential revision: https://reviews.freebsd.org/D31549