This really should be done in mdadm, not mdmon.
We ensure the device won't be suddenly commited as a hot-spare
using O_EXCL, then check the 'holders' sysfs directory
to make sure it is only in use once.
From: Dan Williams <dan.j.williams@intel.com>
1/ Block attempts to add/remove devices from container members
2/ Forward add/remove requests to containers
Signed-off-by: Dan Williams <dan.j.williams@intel.com>