Monitor: refresh mdstat fd after select

Monitor: refresh mdstat fd after select

After 52209d6ee118 ("Monitor: release /proc/mdstat fd when no arrays
present") mdstat fd is closed if mdstat is empty or cannot be opened.
It causes that monitor is not able to select on mdstat. Select
doesn't fail because it gets valid descriptor to a different resource.
As a result any new event will be unnoticed until timeout (delay).

Refresh mdstat after wake up, don't poll on wrong resource.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>