ethdev: support flow elements with variable length

ethdev: support flow elements with variable length

Flow API provides RAW item type for packet patterns of variable
length. The RAW item structure has fixed size members that describe the
variable pattern length and methods to process it.

There is the new Flow items with variable lengths coming - flex
item. In order to handle this item (and potentially other new ones
with variable pattern length) in flow copy and conversion routines
the helper function is introduced.

Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Reviewed-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
Acked-by: Ori Kam <orika@nvidia.com>