External reshape (step 2): Freeze container

External reshape (step 2): Freeze container

When growing the number of raid disks the reshape process will promote
container-spares to subarray-spares (later the kernel promotes them to
subarray-members in raid5_start_reshape()).  The automatic spare
promotion that mdmon performs upon seeing a degraded array must be
disabled until the reshape process has been initiated.  Otherwise, mdmon
may start a rebuild before the reshape parameters can be specified.

In the external case we arrange for the monitor to be blocked, and
turn off the safemode delay.
Mdmon is updated to check sync_action is not frozen before initiating
recovery.

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