osd: fix backoff vs reset race

osd: fix backoff vs reset race

In OSD::ms_handle_reset, we clear session->con before removing any
backoffs.  That means we have to check if con has been cleared after any
call to have_backoff, lest we race with ms_handle_reset and it removes the
backoffs but we don't realize our client session is disconnected.

Introduce a helper to do both these checks in a safe way, simplifying
callers while we're at it.

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