Linux 5.0 compat: Fix bio_set_dev()

The Linux 5.0 kernel updated the bio_set_dev() macro so it calls the
GPL-only bio_associate_blkg() symbol thus inadvertently converting
the entire macro. Provide a minimal version which always assigns the
request queue's root_blkg to the bio.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8287