linux/zvol_os.c: cleanup limits for non-blk mq case

Rob Noris suggested that we could clean up redundant limits for the case
of non-blk mq scenario.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #16462