net: ieee802154: 6lowpan: fix frag reassembly

Hello.

Please apply f18fa5de5ba7f1d6650951502bb96a6e4715a948

(net: ieee802154: 6lowpan: fix frag reassembly) to the 4.16.x stable tree.

Earlier trees are not needed as the problem was introduced in 4.16.

Normally net/ patches would come through DaveM, but he asked me for this one to submit it directly when i sent him the pull request.

First time stable request on my side here, let me know if I got something wrong.

regards

Stefan Schmidt