af_xdp: fix stale rx/tx pointers in xsk objects

xsk objects keep pointers to the rx and tx objects. If we re-allocate
the rx and tx vectors after initializing the associated xsk object, the
pointers in the xsk object will be staled.
To avoid this, we allocate the vectors to the max expected size instead
of growing them.

Type: fix

Change-Id: If30433a28c186787d66c12dbab34bf210c95b519
Signed-off-by: Benoît Ganne <bganne@cisco.com>