ceph: attempt mds reconnect if mds closes our session

Currently, if our session is closed (due to a timeout, or explicit close,
or whatever), we just sit there doing nothing unless/until the MDS
restarts, at which point we try to reconnect.

Change client to attempt an immediate reconnect if our session is closed.

Note that currently the MDS doesn't support this, and our attempt will
fail.  We'll get a session CLOSE, our caps and dirty cap state will be
dropped, and the client will be free to attempt to reconnect.  That's
clearly not as nice as a successful reconnect, but it at least allows us
to try to carry on, and in the future the MDS will support a reconnect
and we will fare better.

Signed-off-by: Sage Weil <sage@newdream.net>