Bluetooth: 6lowpan: Don't hold spin lock over sleeping functions

Bluetooth: 6lowpan: Don't hold spin lock over sleeping functions

disconnect_all_peers() calls sleeping function (l2cap_chan_close) under
spinlock.  Holding the lock doesn't actually do any good -- we work on a
local copy of the list, and the lock doesn't protect against peer->chan
having already been freed.

Fix by taking refcounts of peer->chan instead.  Clean up the code and
old comments a bit.

Take devices_lock instead of RCU, because the kfree_rcu();
l2cap_chan_put(); construct in chan_close_cb() does not guarantee
peer->chan is necessarily valid in RCU.

Also take l2cap_chan_lock() which is required for l2cap_chan_close().

Log: (bluez 6lowpan-tester Client Connect - Disable)
------
BUG: sleeping function called from invalid context at kernel/locking/mutex.c:575
...
<TASK>
...
l2cap_send_disconn_req (net/bluetooth/l2cap_core.c:938 net/bluetooth/l2cap_core.c:1495)
...
? __pfx_l2cap_chan_close (net/bluetooth/l2cap_core.c:809)
do_enable_set (net/bluetooth/6lowpan.c:1048 net/bluetooth/6lowpan.c:1068)
------

Fixes: 90305829635d ("Bluetooth: 6lowpan: Converting rwlocks to use RCU")
Signed-off-by: Pauli Virtanen <pav@iki.fi>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>