libbpf: Always use libbpf_err to return an error in bpf_xdp_query()

In order to properly set errno, rely on libbpf_err utility routine in
bpf_xdp_query() to return an error to the caller.

Fixes: 04d58f1b

 ("libbpf: add API to get XDP/XSK supported features")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/827d40181f9f90fb37702f44328e1614df7c0503.1675768112.git.lorenzo@kernel.org