mds: track created inos with completed request ids in session info

Along with each session completed request (tid), also track the created
ino (if any).  This will be used to pass back to the client when they
replay requests in the next patch.

Do not bother making this a backward compatible encoding.  I am lazy.

While we are here, inline the second add_completed_request() method variant
since there is only a single caller and having it overloaded somewhat
obscures what is going on.  This also avoids a duplicate lookup in the
session map in the have_session() check and then in the (old) helper.

Signed-off-by: Sage Weil <sage@inktank.com>
mds: pass created_ino back to client on replayed requests

After an MDS restart, the client will resend uncommitted requests.  Use the
information we now have in the session_info_t to pass the created ino
back via the extra_bl payload in the reply.

Fixes: #4034
Signed-off-by: Sage Weil <sage@inktank.com>