Bluetooth: Linearize skbs for use in BNEP, CMTP, HIDP, and RFCOMM

Fragmented skbs are only encountered when receiving ERTM or streaming
mode L2CAP data.  BNEP, CMTP, HIDP, and RFCOMM generally use basic
mode, but they need to handle fragments without crashing.

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