Bluetooth: Fix packet size provided to the controller

When building fragmented skb's skb->len keeps track of the size of head
plus all fragments combined, however when queueing the skb for sending we
need to report the head size instead of the total size, so we just set
skb->len to skb_headlen().

This bug appeared when implementing MSG_MORE support for L2CAP sockets, it
never showed up before because l2cap_skbuff_fromiovec() never accounted skb
size correctly. A following patch will fix this.

Signed-off-by: Gustavo Padovan <gustavo@padovan.org>
Reviewed-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>