libbpf: Clean up ringbuf size adjustment implementation

Drop unused iteration variable, move overflow prevention check into the
for loop.

Fixes: 0087a681 ("libbpf: Automatically fix up BPF_MAP_TYPE_RINGBUF size, if necessary")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220510185159.754299-1-andrii@kernel.org