net/mlx4: generalize flow rule priority support

net/mlx4: generalize flow rule priority support

Since both internal and user-defined flow rules are handled by a common
implementation, flow rule priority overlaps are easier to detect. No need
to restrict their use to isolated mode only.

With this patch, only the lowest priority level remains inaccessible to
users outside isolated mode.

Also, the PMD no longer automatically assigns a fixed priority level to
user-defined flow rules, which means collisions between overlapping rules
matching a different number of protocol layers at a given priority level
won't be avoided anymore (e.g. "eth" vs. "eth / ipv4 / udp").

As a reminder, the outcome of overlapping rules for a given priority level
was, and still is, undefined territory according to API documentation.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>