Bluetooth: Handle fragmented skbs in bt_sock_stream_recvmsg()

ERTM reassembly will be more efficient when skbs are linked together
rather than copying every incoming data byte. The existing stream recv
function assumes skbs are linear, so it needs to know how to handle
fragments before reassembly is changed.

bt_sock_recvmsg() already handles fragmented skbs.

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