ceph: cancel delayed work when closing connection

This ensures that if/when we reopen the connection, we can requeue work on
the connection immediately, without waiting for an old timer to expire.
Queue new delayed work inside con->mutex to avoid any race.

This fixes problems with clients failing to reconnect to the MDS due to
the client_reconnect message arriving too late (due to waiting for an old
delayed work timeout to expire).

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