bpf: sync BPF_FIB_LOOKUP flag changes with BPF uapi

Sync the changes to the flags made in "bpf: simplify definition of
BPF_FIB_LOOKUP related flags" with the BPF UAPI headers.

Doing in a separate commit to ease syncing of github/libbpf.

Signed-off-by: Martynas Pumputis <m@lambda.lt>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>