ip: fix IP defragmentation mechanism for big packets

Current limit of fragments number for IP packet is 16.
This is not enough for packets bigger than 24K on standard MTU.
This patch increases this number up to theoretical maximum.

The problem found during UDP RX performance testing - throughput
dropped to 0 for 32K UDP packets.

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>