nlattr: Fix build error with NET off

We moved the netlink attribute support from net to lib in order
for it to be available for general consumption.  However, parts
of the code (the bits that we don't need :) really depends on
NET because the target object is sk_buff.

This patch fixes this by wrapping them in CONFIG_NET.

Some EXPORTs have been moved to make this work.

Tested-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>