tls: rx: don't handle async in tls_sw_advance_skb()

tls_sw_advance_skb() caters to the async case when skb argument
is NULL. In that case it simply unpauses the strparser.

These are surprising semantics to a person reading the code,
and result in higher LoC, so inline the __strp_unpause and
only call tls_sw_advance_skb() when we actually move past
an skb.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>