selftests/bpf: Fix ringbuf test fetching map FD

Seems like 4d1b6298 ("selftests/bpf: Convert few tests to light skeleton.")
and 704e2beb ("selftests/bpf: Test ringbuf mmap read-only and read-write
restrictions") were done independently on bpf and bpf-next trees and are in
conflict with each other, despite a clean merge. Fix fetching of ringbuf's
map_fd to use light skeleton properly.

Fixes: 704e2beb ("selftests/bpf: Test ringbuf mmap read-only and read-write restrictions")
Fixes: 4d1b6298 ("selftests/bpf: Convert few tests to light skeleton.")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210618002824.2081922-1-andrii@kernel.org