mds: open mydir after replay

In certain cases, we may replay the journal and not end up with the
dirfrag for mydir open.  This is fine--we just need to open it up and
fetch it below.

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit e51299fbce6bdc3d6ec736e949ba8643afc965ec)