bpf: Folding omem_charge() into sk_storage_charge()

sk_storage_charge() is the only user of omem_charge().
This patch simplifies it by folding omem_charge() into
sk_storage_charge().

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: KP Singh <kpsingh@google.com>
Link: https://lore.kernel.org/bpf/20201112211301.2586255-1-kafai@fb.com