net/tls: Read sk_prot once when building tls proto ops

Apart from being a "tremendous" win when it comes to generated machine
code (see bloat-o-meter output for x86-64 below) this mainly prepares
ground for annotating access to sk_prot with READ_ONCE, so that we don't
pepper the code with access annotations and needlessly repeat loads.

add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-46 (-46)
Function                                     old     new   delta
tls_init                                     851     805     -46
Total: Before=21063, After=21017, chg -0.22%

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>