bpf, ktls: Fix data corruption when using bpf_msg_pop_data() in ktls

When sending plaintext data, we initially calculated the corresponding
ciphertext length. However, if we later reduced the plaintext data length
via socket policy, we failed to recalculate the ciphertext length.

This results in transmitting buffers containing uninitialized data during
ciphertext transmission.

This causes uninitialized bytes to be appended after a complete
"Application Data" packet, leading to errors on the receiving end when
parsing TLS record.

Fixes: d3b18ad3 ("tls: add bpf support to sk_msg handling")
Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/bpf/20250609020910.397930-2-jiayuan.chen@linux.dev