ping_manager() to prevent 'add' before 'remove' completes

ping_manager() to prevent 'add' before 'remove' completes

It is currently possible to remove a device and re-add it without the
manager noticing, i.e. without detecting a mdstat->devcnt
container->devcnt mismatch.  Introduce ping_manager() to arrange for
mdmon to run manage_container() prior to mdadm dropping the exclusive
open() on the container.  Despite these precautions sysfs_read() may
still fail.  If this happens invalidate container->devcnt to ensure
manage_container() runs at the next event.

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