Bluetooth: Fix deadlock and crash when SMP pairing times out

The l2cap_conn_del function tries to cancel_sync the security timer, but
when it's called from the timeout function itself a deadlock occurs.
Subsequently the "hcon->l2cap_data = NULL" that's supposed to protect
multiple calls to l2cap_conn_del never gets cleared and when the
connection finally drops we double free's etc which will crash the
kernel.

This patch fixes the issue by using the HCI_CONN_LE_SMP_PEND for
protecting against this. The same flag is also used for the same purpose
in other places in the SMP code.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>