ZVOL: Make zvol_volmode module parameter platform-independent

[fuporovvStack]

Motivation and Context

Unify ZFS kernel module parameters names between Linux and FreeBSD.

Description

Make zvol_volmode module parameter unification between Linux and FreeBSD.

The parameter have two entries in the sysctls list on FreeBSD side:

• 'vfs.zfs.vol.mode' - old sysctl name.
• 'vfs.zfs.zvol_volmode' - new one, compatible with Linux.

The sysctl old name is preserved for backward compatibility, if used somewhere by ZFS users or by 3rd party software.

How Has This Been Tested?

No additional tests added.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[fuporovvStack]

@robn, thank you for explanation. I've updated this PR accordingly.
Now the sysctl name on FreeBSD side could be preserved:

$ sysctl vfs.zfs.vol.mode
vfs.zfs.vol.mode: 1

On Linux side, same as you provided above:

/sys/module/zfs/parameters/zvol_volmode 
1

Both other PR's are updated by the same way:
#17384
#17385

Also, I should not forget to fix my previous PR to make all these sysctls compatible:
#17169

[robn]

Looks great, nice work!

[amotin]

I've missed that it failed CI on all Linux'es. We should remove this to not duplicate:

module_param(zvol_volmode, uint, 0644);
MODULE_PARM_DESC(zvol_volmode, "Default volmode property value");