tls: extract context alloc/initialization out of tls_set_sw_offload

[ Upstream commit 615580cb ]

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")
Signed-off-by: Sasha Levin <sashal@kernel.org>