Bluetooth: Perform L2CAP SDU reassembly without copying data

Use sk_buff fragment capabilities to link together incoming skbs
instead of allocating a new skb for reassembly and copying.

The new reassembly code works equally well for ERTM and streaming
mode, so there is now one reassembly function instead of two.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>