Retry HOT_REMOVE_DISK a few times.

Retry HOT_REMOVE_DISK a few times.

HOT_REMOVE_DISK can fail with EBUSY if there are outstanding
IO request that have not completed yet.  It can sometimes
be helpful to wait a little while for these to complete.

We already do this in impose_level() when reshaping a device,
but not in Manage.c in response to an explicit --remove request.

So create hot_remove_disk() to central this code, and call it
where-ever it makes sense to wait for a HOT_REMOVE_DISK to succeed.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@gmail.com>