net/tap: update netlink error code management

net/tap: update netlink error code management

Some errors received from the kernel are acceptable, such as a -ENOENT
for a rule deletion (the rule was already no longer existing in the
kernel). Make sure we consider return codes properly. For that,
nl_recv() has been simplified.

qdisc_exists() function is no longer needed as we can check whether the
kernel returned -EEXIST when requiring the qdisc creation. It's simpler
and faster.

Add a few messages for clarity when a netlink error occurs.

Signed-off-by: Pascal Mazon <pascal.mazon@6wind.com>