net: ip: replace hardcoded NET_IPV6_FRAGMENTS_MAX_PKT with a Kconfig

Currently the stack is limited to a maximum of 2 incoming fragments per
packet. While this can be enough in most cases, it might not be enough
in other cases.

Make this value configurable at build time.

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>