net: Refactor arguments of inet{,6}_bind

The intent is to add an additional bind parameter in the next commit.
Instead of adding another argument, let's convert all existing
flag arguments into an extendable bit field.

No functional changes.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200508174611.228805-4-sdf@google.com