skmsg: Return copied bytes in sk_msg_memcopy_from_iter

Previously sk_msg_memcopy_from_iter returns the copied bytes from the
last copy_from_iter{,_nocache} call upon success.

This commit changes it to return the total number of copied bytes on
success.

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-v1-1-bae583d014f3@outlook.com