6lowpan: move lowpan frag_info out of 802.15.4 headers

Fragmentation and reassembly information for 6lowpan is independent from
the 802.15.4 stack and used only by the 6lowpan reassembly process. Move
the ieee802154_frag_info struct to a private are, it needn't be in the
802.15.4 skb control block.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>