libbpf: Deprecate bpf_map__def() API

All fields accessed via bpf_map_def can now be accessed via
appropirate getters and setters. Mark bpf_map__def() API as deprecated.

Signed-off-by: Christy Lee <christylee@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220108004218.355761-6-christylee@fb.com