attr: mark nested attributes as NLA_F_NESTED

Kernel 5.2 is adding stricter checking for netlink messages.
In particular, for certain API it checks now that NLA_F_NESTED flag is
set for nested attributes ([1]).

Do like libmnl, which always adds this flag ([2]). So we should do
that as well.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b424e432e770d6dd572765459d5b6a96a19c5286
[2] https://git.netfilter.org/libmnl/tree/src/attr.c?id=5937dfcb0185f5cb9cf275992ea701ec4e619d9c#n535

Bug: https://issuetracker.google.com/150744338
Test: treehugger
Change-Id: I9012d820ed72b13d4562e144c48bc7003d621f09