Synchronize unclean mirrors before adding them to a running gmirror.

During gmirror startup, if component mirrors are found to be dirty as is
typical after a system crash, the mirrors are synchronized to the mirror
with highest priority. However if a gmirror starts without all of its
mirrors present, for example because of some transient delays during
tasting, the remaining mirrors must be synchronized before they may become
active.

MFC after: 2 weeks
Sponsored by: Dell EMC Isilon