datapath: Make VERIFY_NUL_STRING verify the string length too.

datapath: Make VERIFY_NUL_STRING verify the string length too.

It's better to use HAVE_NLA_NUL_STRING than a version check because the
Xen 2.6.18 kernels backport NLA_NUL_STRING and the nla_policy changes.

Just defining NLA_NUL_STRING to an innocuous value doesn't work, because
Linux before 2.6.19 doesn't define a 'len' member in struct nla_policy at
all (it was named 'minlen' and had different semantics), so attempting to
initialize it caused compile errors.

Grouping things this way also makes it clearer what needs to be deleted
when upstreaming.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>