mdmon: ignore inactive arrays and other manage_new() cleanups

mdmon: ignore inactive arrays and other manage_new() cleanups

While mdadm is constructing an array mdmon may see an intermediate state
(some disks not yet added / redundancy attributes like sync_action not
available).  Waiting for mdstat->active == true ensures that the array
is ready to be handled.  This fixes a bug in create array via mdmon
update whereby failures are not detected in the new array.

Introduce aa_ready() to catch cases where the active_array is not
correctly initialized.  Barring a kernel bug this should never trigger,
nonetheless it precludes a class of bugs like the one mentioned above
from triggering.

Cleanup the exit paths and only call replace_array when the new array is
ready to be inserted into container->arrays.

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