Merge branch 'bpf-explored-states'

Alexei Starovoitov says:

====================
Convert explored_states array into hash table and use simple hash
to reduce verifier peak memory consumption for programs with bpf2bpf
calls. More details in patch 3.

v1->v2: fixed Jakub's small nit in patch 1
====================

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