net: dcb: add new common function for set/del of app/rewr entries

In preparation for DCB rewrite. Add a new function for setting and
deleting both app and rewrite entries. Moving this into a separate
function reduces duplicate code, as both type of entries requires the
same set of checks. The function will now iterate through a configurable
nested attribute (app or rewrite attr), validate each attribute and call
the appropriate set- or delete function.

Note that this function always checks for nla_len(attr_itr) <
sizeof(struct dcb_app), which was only done in dcbnl_ieee_set and not in
dcbnl_ieee_del prior to this patch. This means, that any userspace tool
that used to shove in data < sizeof(struct dcb_app) would now receive
-ERANGE.

Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>