bpf: Simplify __cgroup_bpf_attach

__cgroup_bpf_attach has a lot of identical code to handle two scenarios:
BPF_F_ALLOW_MULTI is set and unset.

Simplify it by splitting the two main steps:

* First, the decision is made whether a new bpf_prog_list entry should
  be allocated or existing entry should be reused for the new program.
  This decision is saved in replace_pl pointer;

* Next, replace_pl pointer is used to handle both possible states of
  BPF_F_ALLOW_MULTI flag (set / unset) instead of doing similar work for
  them separately.

This splitting, in turn, allows to make further simplifications:

* The check for attaching same program twice in BPF_F_ALLOW_MULTI mode
  can be done before allocating cgroup storage, so that if user tries to
  attach same program twice no alloc/free happens as it was before;

* pl_was_allocated becomes redundant so it's removed.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/c6193db6fe630797110b0d3ff06c125d093b834c.1576741281.git.rdna@fb.com