zvol: make calls to platform ops static

There's no need to make the platform ops dynamic dispatch.

This change replaces the dynamic dispatch with static calls to the
platform-specific functions.
To avoid name collisions, prefix all platform-specific functions
with zvol_os_.
I actually find zvol_..._os slightly nicer to read in the calling
code, but having it as a prefix is useful.

Advantage:

• easier jump-to-definition / grepping
• potential benefits to static analysis
• better legibility

Future work: also prefix remaining static functions in zvol_os.c.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Christian Schwarz <christian.schwarz@nutanix.com>
Closes #12965