bpf: Add bpf_sk_storage support to bpf_tcp_ca

This patch adds bpf_sk_storage_get() and bpf_sk_storage_delete()
helper to the bpf_tcp_ca's struct_ops.  That would allow
bpf-tcp-cc to:
1) share sk private data with other bpf progs.
2) use bpf_sk_storage as a private storage for a bpf-tcp-cc
   if the existing icsk_ca_priv is not big enough.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200320152101.2169498-1-kafai@fb.com