tcp: shrink per-packet memset in __tcp_transmit_skb()

tcp: shrink per-packet memset in __tcp_transmit_skb()

Use struct_group() to group the three fields in tcp_out_options that are
read unconditionally by tcp_options_write() and bpf_skops_write_hdr_opt()
(mss, bpf_opt_len, num_sack_blocks), then replace the full-struct memset
with a targeted memset of only that group.

struct tcp_out_options is 40 bytes without MPTCP and 96 bytes with
CONFIG_MPTCP=y (typical distro config). Every remaining field is either
assigned before first use by tcp_established_options()/tcp_syn_options(),
or gated behind its OPTION_* flag in tcp_options_write(). This memset
runs on every transmitted TCP packet, so shrinking it from 96 (or 40)
bytes to 4 bytes reduces per-packet overhead on the hot path.

Assembly comparison (x86-64, GCC 13, CONFIG_MPTCP=y):

  Before: rep stos zeroing 96 bytes (5 instructions, 12 8-byte stores)
  After:  movl $0x0 zeroing 4 bytes (1 instruction, 1 store)

Also add opts->options = 0 at the top of tcp_syn_options(), which
already used |= without a prior clear. tcp_established_options() already
clears opts->options at its top.

Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Keita Morisaki <kmta1236@gmail.com>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260304111517.2088694-1-kmta1236@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>