mdmon: avoid writes in the startup path for mdmon on root arrays

mdmon: avoid writes in the startup path for mdmon on root arrays

When killing a previous monitor be careful not to cause writes to the
filesystem until the reads necessary to get the monitor operational have
completed.

The code is already prepared for errors creating the pid and socket
files, so simply defer creation of these files until after the first
call to manage().

Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>