9539 Make zvol operations use _by_dnode routines

9539 Make zvol operations use _by_dnode routines

Continues what was started in 7801 add more by-dnode routines 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.

illumos/illumos-gate@8dfe5547fbf0979fc1065a8b6fddc1e940a7cf4f

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Rick McNeal <rick.mcneal@nexenta.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author:     Richard Yao <richard.yao@prophetstor.com>