It may happen, though rarely, that frames are not sent (or not freed
at kernel level), and we build up an output queue, so at some point we
get -EAGAIN on sending.

With blocking writes, this situation was stalling the whole process.
This patch helped us diagnosing a problem in our own network driver,
which had a memory leakage, but using non-blocking sockets and
reporting erors is the right thing to do anyways.

This patch reports the error using pp_diag with a priority 0; thus
the message is always printed, but by virtue of pp_diag we have the
name of the port automatically added.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>