The NLMSGERR_ATTR_POLICY extack attribute has been ignored by ynl up to
now. Extend extack decoding to include _POLICY and the nested
NL_POLICY_TYPE_ATTR_* attributes.

For example:

./tools/net/ynl/cli.py \
  --spec Documentation/netlink/specs/rt_link.yaml \
  --create --do newlink --json '{
    "ifname": "12345678901234567890",
    "linkinfo": {"kind": "bridge"}
    }'
Netlink error: Numerical result out of range
nl_len = 104 (88) nl_flags = 0x300 nl_type = 2
	error: -34	extack: {'msg': 'Attribute failed policy validation',
'policy': {'max-length': 15, 'type': 'string'}, 'bad-attr': '.ifname'}
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240328155636.64688-1-donald.hunter@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>