Merge branch 'libbpf: support non-mmap()'able data sections'

Andrii Nakryiko says:

====================

Make libbpf more conservative in using BPF_F_MMAPABLE flag with internal BPF
array maps that are backing global data sections. See patch #2 for full
description and justification.

Changes in this dataset support having bpf_spinlock, kptr, rb_tree nodes and
other "special" variables as global variables. Combining this with libbpf's
existing support for multiple custom .data.* sections allows BPF programs to
utilize multiple spinlock/rbtree_node/kptr variables in a pretty natural way
by just putting all such variables into separate data sections (and thus ARRAY
maps).

v1->v2:
  - address Stanislav's feedback, adds acks.
====================

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>