Bluetooth: bnep: Fix deadlock in session deletion

Commit f4d7cd4a introduced the usage of kthread API.
kthread_stop is a blocking function which returns only when
the thread exits. In this case, the thread can't exit because it's
waiting for the write lock, which is being held by bnep_del_connection()
which is waiting for the thread to exit -- deadlock.

Use atomic_t/wake_up_process instead to signal to the thread to exit.

Signed-off-by: Jaikumar Ganesh <jaikumar@google.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>