libbpf: Use __u32 fields in bpf_map_create_opts

Corresponding Linux UAPI struct uses __u32, not int, so keep it
consistent.

Fixes: 992c4225

 ("libbpf: Unify low-level map creation APIs w/ new bpf_map_create()")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211201232824.3166325-2-andrii@kernel.org