net: dcb: set error code on failures

In function dcbnl_cee_fill(), returns the value of variable err on
errors. However, on some error paths (e.g. nla put fails), its value may
be 0. It may be better to explicitly set a negative errno to variable
err before returning.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=188881



Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: David S. Miller <davem@davemloft.net>