osd: fix hb con failure handler

- 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.

This fixes the problem where we reopen the front con before closing it and
get back the same one.. and then close it again.

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