Bluetooth: Lock sockets when closing HCI device

When a Bluetooth device is powered down while RFCOMM connections are
open, there are race conditions where RFCOMM tries to use an HCI
connection after the data structures have been deleted.  Since the
powerdown procedure runs in process context, it is possible to lock
sockets before accessing them to close connections.  This change adds
that locking, which will serialize socket access between processors
during powerdown.

Change-Id: I587720799b35d9ee6e66340bdeda8a2b9e2ea7b5
CRs-fixed: 336893
Signed-off-by: Mat Martineau <mathewm@codeaurora.org>