Merge branch 'bpf: Enable bpf_sk_storage for FENTRY/FEXIT/RAW_TP'

Martin KaFai says:

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

This set is to allow the FENTRY/FEXIT/RAW_TP tracing program to use
bpf_sk_storage.  The first two patches are a cleanup.  The last patch is
tests.  Patch 3 has the required kernel changes to
enable bpf_sk_storage for FENTRY/FEXIT/RAW_TP.

Please see individual patch for details.

v2:
- Rename some of the function prefix from sk_storage to bpf_sk_storage
- Use prefix check instead of substr check
====================

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