IFLA_LINK is no longer a write-only attribute on the kernel side and
must thus be validated. Same goes for the newly introduced
IFLA_LINKINFO.

Fixes undefined behaviour if either of the attributes are not well
formed.
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>