Rename zvol kernel module parameters sysctls on FreeBSD side

[fuporovvStack]

Motivation and Context

Make modules parameters names different for FreeBSD and Linux.

Description

Make 'zvol_threads', 'zvol_num_taskqs' and 'zvol_request_sync' names compatible with FreeBSD sysctl naming convention. Now the sysctls are have a next form:

$ sysctl vfs.zfs.vol.threads
vfs.zfs.vol.threads: 0

$ sysctl vfs.zfs.vol.num_taskqs
vfs.zfs.vol.num_taskqs: 0

$ sysctl vfs.zfs.vol.request_sync
vfs.zfs.vol.request_sync: **0**

How Has This Been Tested?

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.