tcp_bpf: Fix copied value in tcp_bpf_sendmsg

bpf kselftest sockhash::test_txmsg_cork_hangs in test_sockmap.c triggers a
kernel NULL pointer dereference:

BUG: kernel NULL pointer dereference, address: 0000000000000008
 ? __die_body+0x6e/0xb0
 ? __die+0x8b/0xa0
 ? page_fault_oops+0x358/0x3c0
 ? local_clock+0x19/0x30
 ? lock_release+0x11b/0x440
 ? kernelmode_fixup_or_oops+0x54/0x60
 ? __bad_area_nosemaphore+0x4f/0x210
 ? mmap_read_unlock+0x13/0x30
 ? bad_area_nosemaphore+0x16/0x20
 ? do_user_addr_fault+0x6fd/0x740
 ? prb_read_valid+0x1d/0x30
 ? exc_page_fault+0x55/0xd0
 ? asm_exc_page_fault+0x2b/0x30
 ? splice_to_socket+0x52e/0x630
 ? shmem_file_splice_read+0x2b1/0x310
 direct_splice_actor+0x47/0x70
 splice_direct_to_actor+0x133/0x300
 ? do_splice_direct+0x90/0x90
 do_splice_direct+0x64/0x90
 ? __ia32_sys_tee+0x30/0x30
 do_sendfile+0x214/0x300
 __se_sys_sendfile64+0x8e/0xb0
 __x64_sys_sendfile64+0x25/0x30
 x64_sys_call+0xb82/0x2840
 do_syscall_64+0x75/0x110
 entry_SYSCALL_64_after_hwframe+0x4b/0x53

This is caused by tcp_bpf_sendmsg() returning a larger value(12289) than
size (8192), which causes the while loop in splice_to_socket() to release
an uninitialized pipe buf.

The underlying cause is that this code assumes sk_msg_memcopy_from_iter()
will copy all bytes upon success but it actually might only copy part of
it.

This commit changes it to use the real copied bytes.

Signed-off-by: Levi Zim <rsworktech@outlook.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Björn Töpel <bjorn@kernel.org>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20241130-tcp-bpf-sendmsg-v1-2-bae583d014f3@outlook.com