When the vdev properties features was merged an extra check
was added in `spa_vdev_remove_top_check()` which checked
whether the vdev that we want to remove is already being
removed and if so return an EALREADY error.

```
static int
spa_vdev_remove_top_check(vdev_t *vd)
{
	... <snip> ...
	/*
	 * This device is already being removed
	 */
	if (vd->vdev_removing)
		return (SET_ERROR(EALREADY));
```

Before that change we'd still fail with an error but it
was a more generic one - here is the check that failed
later in the same function:
```
	/*
	 * There can not be a removal in progress.
	 */
	if (spa->spa_removing_phys.sr_state == DSS_SCANNING)
		return (SET_ERROR(EBUSY));
```

Changing the error code returned from that function changed
the behavior of the removal's library interface exposed to
the userland - `spa_vdev_remove()` now returns `EZFS_UNKNOWN`
instead of `EZFS_EBUSY` that was returning before.

This patch adds logic to make `spa_vdev_remove()` mindful
of the new EALREADY code and propagating `EZFS_EBUSY`
reverting to the previously established semantics of that
function.
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #15013
Closes #15129