bpf: Avoid ABI change in 4.9.77

bpf: Avoid ABI change in 4.9.77

Commit a9bfac14cde2 "bpf: prevent out-of-bounds speculation" added one
member each to struct bpf_map and struct bpf_array (which is
effectively a sub-type of bpf_map).  Changing the size of struct
bpf_array is an ABI change, since the array contents immediately
follows the structure.  However, bpf_map::work is not used (or even
initialised) until after the map's refcount drops to zero.  We can
therefore move the new members into a union with it.

Based on the patch against 4.14.14 by Ben Hutchings <ben@decadent.org.uk>

Gbp-Pq: Topic debian
Gbp-Pq: Name bpf-avoid-abi-change-in-4.9.77.patch