tls: extract context alloc/initialization out of tls_set_sw_offload

commit 615580cb upstream.

Simplify tls_set_sw_offload a bit.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: aec79619 ("tls: fix race between async notify and socket close")
[v5.15: fixed contextual conflicts from unavailable init_waitqueue_head and
skb_queue_head_init calls in tls_set_sw_offload and init_ctx_rx]
Cc: <stable@vger.kernel.org> # 5.15
Signed-off-by: Shaoying Xu <shaoyi@amazon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>