Bluetooth: Introduce L2CAP_LM_FLUSHABLE to allow flushing of ACL packets.

Bluetooth: Introduce L2CAP_LM_FLUSHABLE to allow flushing of ACL packets.

With Bluetooth 2.1 ACL packets can be flushable or non-flushable. This changes
makes the default ACL packet non-flushable, and allows selection of flushable
packets on a per-L2CAP socket basis with L2CAP_LM_FLUSHABLE.

Note the HCI Write Automatic Flush Timeout command also needs to be issued
to set the flush timeout to non-zero.

Need to featurize this change to Bluetooth 2.1 chipsets only before pushing
upstream.

Signed-off-by: Nick Pelly <npelly@google.com>