bpf: Fix inaccurate helper function description

The description inside uapi/linux/bpf.h about bpf_get_socket_uid
helper function is no longer valid. It returns overflowuid rather
than 0 when failed.

Signed-off-by: Chenbo Feng <fengc@google.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>