libceph: use a do..while loop in con_work()

This just converts a manually-implemented loop into a do..while loop
in con_work().  It also moves handling of EAGAIN inside the blocks
where it's already been determined an error code was returned.

Also update a few dout() calls near the affected code for
consistency.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>