Bluetooth: Fix for the l2cap layer crashes while turning OFF BT

-This patch fix a case where while turning OFF BT l2cap channel
 & l2cap conn get deleted in one process context, which preempted
 the another process, where l2cap con was being used while trying to
 disconnect l2cap.  As the result l2cap conn pointer become invalid.
 Later which is leading a crash.  L2cap conn should be guarded by
 spin lock while disconnecting l2cap to avoid preemption which will
 avoid  crash due invalid conn pointer.

-This patch fix list_add crash due to invalid skb pointer.
 & BUG sleep warning as well.

CRs-Fixed: 499732, 504054

Change-Id: I90c5d29cc8728566a420f0dedaf7075846b32d1a
Signed-off-by: Nitin Shivpure <nshivpur@codeaurora.org>