tls: avoid field-spanning write when copying crypto info

do_tls_setsockopt_conf() copies the cipher-specific part of the crypto
info to crypto_info + 1, where crypto_info points at the 4-byte info
member of union tls_crypto_context. When optval is a kernel sockptr,
copy_from_sockptr_offset() is a memcpy(), and FORTIFY_SOURCE bounds the
destination by that member:

  [    6.948348] memcpy: detected field-spanning write (size 52) of single field "dst" at include/linux/sockptr.h:49 (size 0)
  [    6.948480] WARNING: include/linux/sockptr.h:49 at tls_setsockopt+0x9b2/0xa30, CPU#1: init/1
  [    7.001355] Call Trace:
  [    7.003146]  do_sock_setsockopt+0xb0/0x190
  [    7.003975]  __sys_setsockopt+0x76/0xc0
  [    7.005368]  do_syscall_64+0xda/0x4b0

tls_setsockopt() gets a kernel sockptr whenever a cgroup setsockopt BPF
program is attached, even one that leaves the option alone, because
__cgroup_bpf_run_filter_setsockopt() passes optval on in a kernel
buffer. With a no-op program on the root cgroup, a plain TLS_TX
setsockopt() with AES-GCM-256 triggers the warning above on an x86_64
build with CONFIG_FORTIFY_SOURCE.

Copy into the union at offset sizeof(struct tls_crypto_info) instead.
crypto_ctx already points at the right union for both the first set
and the TLS 1.3 rekey (tmp) case, and the address is unchanged.

Fixes: a7b75c5a8c41 ("net: pass a sockptr_t into ->setsockopt")
Signed-off-by: Nandor Kracser <nandor@riptides.io>
Link: https://patch.msgid.link/20261021073233.16720-1-nandor@riptides.io
Signed-off-by: Jakub Kicinski <kuba@kernel.org>