mbuf: enforce no option for dynamic fields and flags

mbuf: enforce no option for dynamic fields and flags

As stated in the API, dynamic field and flags should be created with no
additional flag (simply in the API for future changes).

Fix the dynamic flag register helper which was not enforcing it and add
unit tests.

Fixes: 4958ca3a443a ("mbuf: support dynamic fields and flags")

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Ray Kinsella <mdr@ashroe.eu>