Fix readonly check for vdev user properties

[amotin]

VDEV_PROP_USERPROP is equal do VDEV_PROP_INVAL and so is not a real property. That's why vdev_prop_readonly() does not work right for it. In particular it may declare all vdev user properties readonly on FreeBSD.

How Has This Been Tested?

Before:

# zpool set cool:pool=goodbye hdd da0p2
cannot set property cool:pool for da0p2 on hdd: pool is read-only

After:

# zpool set cool:pool=goodbye hdd da0p2
# zpool get cool:pool hdd da0p2
NAME        PROPERTY   VALUE      SOURCE
da0p2       cool:pool  goodbye    local

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.

[robn]

Looks right. Added to #16887 for the test suite run.