tls: skip setting sk_write_space on rekey

tls: skip setting sk_write_space on rekey

syzbot reported a problem when calling setsockopt(SO_SNDBUF) after a
rekey. SO_SNDBUF calls sk_write_space, ie tls_write_space, which then
calls the original socket's sk_write_space, saved in
ctx->sk_write_space. Rekeys should skip re-assigning
ctx->sk_write_space, so we don't end up with tls_write_space calling
itself.

Fixes: 47069594e67e ("tls: implement rekey for TLS1.3")
Reported-by: syzbot+6ac73b3abf1b598863fa@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/676d231b.050a0220.2f3838.0461.GAE@google.com/
Tested-by: syzbot+6ac73b3abf1b598863fa@syzkaller.appspotmail.com
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/ffdbe4de691d1c1eead556bbf42e33ae215304a7.1736436785.git.sd@queasysnail.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>