selftests/bpf: Add apply_bytes test to test_txmsg_redir_wait_sndmem in test_sockmap

Add this to more comprehensively test the socket memory accounting logic
in the __SK_REDIRECT and __SK_DROP cases of tcp_bpf_sendmsg. We don't have
test when apply_bytes are not zero in test_txmsg_redir_wait_sndmem.
test_send_large has opt->rate=2, it will invoke sendmsg two times.

Specifically, the first sendmsg will trigger the case where the ret value
of tcp_bpf_sendmsg_redir is less than 0; while the second sendmsg happens
after the 3 seconds timeout, and it will trigger __SK_DROP because socket
c2 has been removed from the sockmap/hash.

Signed-off-by: Zijian Zhang <zijianzhang@bytedance.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20241016234838.3167769-2-zijianzhang@bytedance.com