osd: make ms_handle_reset debug more useful

Signed-off-by: Sage Weil <sage@inktank.com>
msgr: generate reset event on mark_down to addr (not con)

If the caller is marking down an addr, they presumably don't have the
Connection* handy, so we should generate a reset event to help them
clean up con <-> session ref cycles.

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