MD: Long delay for container drive removal

[Mateusz Kusiak <mateusz.kusiak@linux.intel.com>]

Hi all,
we have an issue submitted for SLES15SP6 that is caused by huge delays when trying to remove drive 
from a container.

The scenario is as follows:
1. Create two drive imsm container
# mdadm --create --run /dev/md/imsm --metadata=imsm --raid-devices=2 /dev/nvme[0-1]n1
2. Remove single drive from container
# mdadm /dev/md127 --remove /dev/nvme0n1

The problem is that drive removal may take up to 7 seconds, which causes timeouts for other 
components that are mdadm dependent.

We narrowed it down to be MD related. We tested this with inbox mdadm-4.3 and mdadm-4.2 on SP6 and 
delay time is pretty much the same. SP5 is free of this issue.

I also tried RHEL 8.9 and drive removal is almost instant.

Is it default behavior now, or should we treat this as an issue?

Thanks,
Mateusz