Enable /proc/diskstats for zvols

The /proc/diskstats accounting needs to be explicitly enabled
for block devices which do not use multi-queue.

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12440
Closes #12066