FreeBSD: Fix scope of deadman tunables

[ghost]

Motivation and Context

A couple of deadman tunables ended up in the wrong sysctl node.

Description

Move them to vfs.zfs.deadman.*, fix ZTS tunables.cfg where a couple other deadman tunables were misnamed as well.
I've intentionally left vfs.zfs.zio.deadman_log_all where it is, to not stray too far from the documented tunable names.

How Has This Been Tested?

Before:

$ sysctl vfs.zfs | grep deadman
vfs.zfs.deadman_enabled: 1
vfs.zfs.deadman_checktime_ms: 60000
vfs.zfs.zio.deadman_log_all: 0
vfs.zfs.deadman.ziotime_ms: 300000
vfs.zfs.deadman.synctime_ms: 600000
vfs.zfs.deadman.failmode: wait

After:

$ sysctl vfs.zfs | grep deadman
vfs.zfs.zio.deadman_log_all: 0
vfs.zfs.deadman.ziotime_ms: 300000
vfs.zfs.deadman.synctime_ms: 600000
vfs.zfs.deadman.failmode: wait
vfs.zfs.deadman.enabled: 1
vfs.zfs.deadman.checktime_ms: 60000

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.

[amotin]

This is more of a creating new consistency rather than restoring previous one. I'd probably just restore the previous, but I don't insist. I am OK with any way, but would be good to have a wider look to see what is better.