Make more taskq parameters writable

[amotin]

There is no reason for these module parameters to be read-only. Being modified they just apply on next pool import/creation, that is useful for testing different values.

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)
• 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.

[rincebrain]

It's probably useful to call out the fact that those only change for pools imported after you modify them, lest people start asking why their changes didn't work. (That is, in the man page for these, since before it wasn't possible to observe this fact.)

[amotin]

The more we write, the less people read...

[rincebrain]

I mean, yes, but most tunables we call out if they don't apply "normally", like the scrub tunables not applying to existing scrubs.

[behlendorf]

Yes, if we're going to make these runtime tunable let's document when those updated values get applied. I'd think we'd also want to make the same change for zio_taskq_read and zio_taskq_write. I see zio_taskq_wr_iss_ncpus can already be modified and that's not documented either.

It would take a bit more work but it would be really nice for performance testing if the taskqs could be recreated, or modified, immediately.

[amotin]

@rincebrain @behlendorf Updated.