Make zvol operations use _by_dnode routines

This continues what was started in
0eef1bde31d67091d3deed23fe2394f5a8bf2276 by fully converting zvols
to avoid unnecessary dnode_hold() calls. This saves a small amount
of CPU time and slightly improves latencies of operations on zvols.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@prophetstor.com>
Closes #6058