bpf: Fix bpf_ringbuf_output() signature to return long

Due to bpf tree fix merge, bpf_ringbuf_output() signature ended up with int as
a return type, while all other helpers got converted to returning long. So fix
it in bpf-next now.

Fixes: b0659d8a

 ("bpf: Fix definition of bpf_ringbuf_output() helper in UAPI comments")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200727224715.652037-1-andriin@fb.com