CAPI: Fix locking around capiminor's output queue and drop workaround_lock

Introduce outlock as a spin lock that protects capiminor's outqueue,
outbytes and outskb (formerly known as ttyskb). outlock can be acquired
from soft-IRQ context via capinc_write, so make it bh-safe.

This finally removes the last reason for keeping the workaround lock
around (which was incomplete and partly broken anyway). And as we no
longer call handle_recv_skb in atomic context, gen_data_b3_resp_for can
use non-atomic allocation now.

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: David S. Miller <davem@davemloft.net>