mdmon: cleanup resync_start

mdmon: cleanup resync_start

We don't need to sprinkle reads of this attribute all over the place,
just once at the entry of read_and_act().  Also, the mdinfo structure
for the array already has a 'resync_start' member, so just reuse that.
Finally, rename get_resync_start() to read_resync_start to make it
consistent with the other sysfs accessors in monitor.c.

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