Prevent NULL pointer dereference in blkg_tryget() on EL8 kernels

blkg_tryget() as shipped in EL8 kernels does not seem to handle NULL
@blkg as input; this is different from its mainline counterpart where
NULL is accepted. To prevent dereferencing a NULL pointer when dealing
with block devices which do not set a root_blkg on the request queue
perform the NULL check in vdev_bio_associate_blkg().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #9546
Closes #9577