net: ipv6: Make max number of fragmented pkt configurable

If the user really wants, it is possible to increase the
maximum size of the fragmented packet. According to RFC 2460
chapter 5, we do not need to accept larger than 1500 byte IPv6
packets, so the max pkt limit is set to 2. But if really needed
the limit can be raised by defining NET_IPV6_FRAGMENTS_MAX_PKT
to some new value. Currently there is no Kconfig option for
doing this as it is unlikely that this is needed.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>