osd: fix hb con failure handler

Fix a few bugs introduced by 27381c0c6259ac89f5f9c592b4bfb585937a1cfc:

- check against both front and back cons; either one may have failed.
- close *both* front and back before reopening either.  this is
  overkill, but slightly simpler code.
- fix leak of con when marking down
- handle race against osdmap update and note_down_osd

Fixes: #5172
Signed-off-by: Sage Weil <sage@inktank.com>
osd: fix note_down_osd

Fix bug introduced in 27381c0c6259ac89f5f9c592b4bfb585937a1cfc.

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