ip/defrag: fix processing of last short fragment

TFTP'ing a file of size 1747851 bytes with CONFIG_IP_DEFRAG and
CONFIG_TFTP_BLOCKSIZE set to 4096 fails with a timeout, because
the last fragment is not taken into account. This patch fixes
IP fragments having less than 8 bytes of payload.

Signed-off-by: Stephane Fillod <stephane.fillod@grassvalley.com>
Acked-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>