Add explicit error for device_rebuild being disabled

[rincebrain]

Motivation and Context

(I ran into #11414, that's really all.)

Currently, you get back "can only attach to mirrors and top-level disks"
unconditionally if zpool attach returns ENOTSUP, but that also happens
if, say, feature@device_rebuild=disabled and you tried attach -s.

Description

Adds an error case for ENOTSUP and feature@device_rebuild=disabled.

(I don't check for unsupported explicitly because it's checked earlier in the function; I considered adding this error check right below that one, but opted to put it with the ENOTSUP return handling rather than another special case early on. If people disagree strongly, I can move it.)

How Has This Been Tested?

$ sudo zpool attach -s ephemeral ata-HGST_HUH728080ALE604_1234 /badideatest
cannot attach /badideatest to ata-HGST_HUH728080ALE604_1234: can only attach to mirrors and top-level disks
$ sudo cmd/zpool/zpool attach -s ephemeral ata-HGST_HUH728080ALE604_2EGTR1JX /badideatest
cannot attach /badideatest to ata-HGST_HUH728080ALE604_1234: device_rebuild feature must be enabled in order to use sequential reconstruction
$ sudo zpool get feature@device_rebuild ephemeral
NAME       PROPERTY                VALUE                   SOURCE
ephemeral  feature@device_rebuild  disabled                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.

[behlendorf]

Handling it here seems fine to me. Thanks for the fix.