mdmon: fix wrong array state when disk fails during mdmon startup

mdmon: fix wrong array state when disk fails during mdmon startup

If a member drive disappears and is set faulty by the kernel during
mdmon startup, after ss->load_container() but before manage_new(), mdmon
will try to readd the faulty drive to the array and start rebuilding.
Metadata on the active drive is updated, but the faulty drive is not
removed from the array and is left in a "blocked" state and any write
request to the array will block. If the faulty drive reappears in the
system e.g. after a reboot, the array will not assemble because metadata
on the drives will be incompatible (at least on imsm).

Fix this by adding a new option for sysfs_read(): "GET_DEVS_ALL". This
is an extension for the "GET_DEVS" option and causes all member devices
to be returned, even if the associated block device has been removed.
Use this option in manage_new() to include the faulty device on the
active_array's devices list. Mdmon will then properly remove the faulty
device from the array and update the metadata to reflect the degraded
state.

Signed-off-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>