netlink: add support for ext_ack missing attributes

There is currently no way to report via extack in a structured way
that an attribute is missing. This leads to families resorting to
string messages.

Add a pair of attributes - @offset and @type for machine-readable
way of reporting missing attributes. The @offset points to the
nest which should have contained the attribute, @type is the
expected nla_type. The offset will be skipped if the attribute
is missing at the message level rather than inside a nest.

User space should be able to figure out which attribute enum
(AKA attribute space AKA attribute set) the nest pointed to by
@offset is using.

Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/userspace-api/netlink/intro.rst

@@ -359,8 +359,8 @@ compatibility this feature has to be explicitly enabled by setting
 the ``NETLINK_EXT_ACK`` setsockopt() to ``1``.
 
 Types of extended ack attributes are defined in enum nlmsgerr_attrs.
-The two most commonly used attributes are ``NLMSGERR_ATTR_MSG``
-and ``NLMSGERR_ATTR_OFFS``.
+The most commonly used attributes are ``NLMSGERR_ATTR_MSG``,
+``NLMSGERR_ATTR_OFFS`` and ``NLMSGERR_ATTR_MISS_*``.
 
 ``NLMSGERR_ATTR_MSG`` carries a message in English describing
 the encountered problem. These messages are far more detailed
@@ -368,6 +368,9 @@ than what can be expressed thru standard UNIX error codes.
 
 ``NLMSGERR_ATTR_OFFS`` points to the attribute which caused the problem.
 
+``NLMSGERR_ATTR_MISS_TYPE`` and ``NLMSGERR_ATTR_MISS_NEST``
+inform about a missing attribute.
+
 Extended ACKs can be reported on errors as well as in case of success.
 The latter should be treated as a warning.