odp-util: Accept fields with zero mask

It is possible to pass some fields to the kernel with a zero mask, but
ovs-dpctl doesn't currently allow it. Change the code to allow it to
mimic what vswitchd is allowed to do.

Signed-off-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Jesse Gross <jesse@kernel.org>