Not all block devices, notably scsi_debug, set a root_blkg on the
request queue. Remove this assertion and allow the the existing
call to blkg_tryget() to gracefully handle the NULL (which it does).
Reviewed-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#8678
c12ea77865