md: disconnect device from personality before trying to remove it.

When the HOT_REMOVE_DISK ioctl is used to remove a device, we
call remove_and_add_spares() which will remove it from the personality
if possible.  This improves the chances that the removal will succeed.

When writing "remove" to dev-XX/state, we don't.  So that can fail more easily.

So add the remove_and_add_spares() into "remove" handling.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>