Bluetooth: Avoid rfcomm_session_timeout using freed session
Use del_timer_sync() instead of del_timer() as this ensures
that rfcomm_session_timeout() is not running on a different
CPU when rfcomm_session_put() is called. This avoids a race
condition on SMP systems because potentially
rfcomm_session_timeout() could reuse the freed RFCOMM session
structure caused by the execution of rfcomm_session_put().
Note that this modification makes the reason for the RFCOMM
session refcnt mechanism redundant.