Merge branch 'Add bpf_skc_to_unix_sock() helper'

Hengqi Chen says:

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

This patch set adds a new BPF helper bpf_skc_to_unix_sock().
The helper is used in tracing programs to cast a socket
pointer to a unix_sock pointer.

v2->v3:
 - Use abstract socket in selftest (Alexei)
 - Run checkpatch script over patches (Andrii)

v1->v2:
 - Update selftest, remove trailing spaces changes (Song)
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>