bpf: Rename some functions in bpf_sk_storage

Rename some of the functions currently prefixed with sk_storage
to bpf_sk_storage.  That will make the next patch have fewer
prefix check and also bring the bpf_sk_storage.c to a more
consistent function naming.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: KP Singh <kpsingh@google.com>
Link: https://lore.kernel.org/bpf/20201112211307.2587021-1-kafai@fb.com