bpf: Rename bpf_htab to bpf_shtab in sock_map

There are two different `struct bpf_htab` in bpf code in the following
files:
- kernel/bpf/hashtab.c
- net/core/sock_map.c

It makes it impossible to find proper btf_id by name = "bpf_htab" and
kind = BTF_KIND_STRUCT what is needed to support access to map ptr so
that bpf program can access `struct bpf_htab` fields.

To make it possible one of the struct-s should be renamed, sock_map.c
looks like a better candidate for rename since it's specialized version
of hashtab.

Rename it to bpf_shtab ("sh" stands for Sock Hash).

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/c006a639e03c64ca50fc87c4bb627e0bfba90f4e.1592600985.git.rdna@fb.com