seg6: add netlink_ext_ack support in parsing SRv6 behavior attributes

An SRv6 behavior instance can be set up using mandatory and/or optional
attributes.
In the setup phase, each supplied attribute is parsed and processed. If
the parsing operation fails, the creation of the behavior instance stops
and an error number/code is reported to the user.  In many cases, it is
challenging for the user to figure out exactly what happened by relying
only on the error code.

For this reason, we add the support for netlink_ext_ack in parsing SRv6
behavior attributes. In this way, when an SRv6 behavior attribute is
parsed and an error occurs, the kernel can send a message to the
userspace describing the error through a meaningful text message in
addition to the classic error code.

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>