ZVOL: Make zvol_prefetch_bytes module parameter platform-independent

[fuporovvStack]

Motivation and Context

Unify ZFS kernel module parameters names between Linux and FreeBSD.

Description

The zvol_prefetch_bytes module parameter now is represented in FreeBSD sysctls list with name: 'vfs.zfs.zvol_prefetch_bytes'. The default value is 131072, same as on Linux side.

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]

Make sysctl name more compatible to zvol sysctls subgroup on FreeBSD side.
Now it looks like:

$ sysctl vfs.zfs.vol.prefetch_bytes
vfs.zfs.vol.prefetch_bytes: 131072

Also, on Linux side:

$ cat /sys/module/zfs/parameters/zvol_prefetch_bytes 
131072