libbpf: Simplify the find_elf_sec_sz() function

The check in the last return statement is unnecessary, we can just return
the ret variable.

But we can simplify the function further by returning 0 immediately if we
find the section size and -ENOENT otherwise.

Thus we can also remove the ret variable.

Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220223085244.3058118-1-ytcoode@gmail.com