Assemble: block attempts to reassemble container members

Assemble: block attempts to reassemble container members

Attempting to open(O_EXCL) each candidate device usually filters out all
busy raid components.  However, containers do not behave like components
and will return container_content that may describe active member
arrays.

This patch just adds a function that will be used to check if a
container member is busy.  It will be used shortly.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>