bpf: Improve bucket_log calculation logic

It was reported that the max_t, ilog2, and roundup_pow_of_two macros have
exponential effects on the number of states in the sparse checker.

This patch breaks them up by calculating the "nbuckets" first so that the
"bucket_log" only needs to take ilog2().

In addition, Linus mentioned:

  Patch looks good, but I'd like to point out that it's not just sparse.

  You can see it with a simple

    make net/core/bpf_sk_storage.i
    grep 'smap->bucket_log = ' net/core/bpf_sk_storage.i | wc

  and see the end result:

      1  365071 2686974

  That's one line (the assignment line) that is 2,686,974 characters in
  length.

  Now, sparse does happen to react particularly badly to that (I didn't
  look to why, but I suspect it's just that evaluating all the types
  that don't actually ever end up getting used ends up being much more
  expensive than it should be), but I bet it's not good for gcc either.

Fixes: 6ac99e8f ("bpf: Introduce bpf sk local storage")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Link: https://lore.kernel.org/bpf/20200207081810.3918919-1-kafai@fb.com

net/core/bpf_sk_storage.c

@@ -643,9 +643,10 @@ static struct bpf_map *bpf_sk_storage_map_alloc(union bpf_attr *attr)
 		return ERR_PTR(-ENOMEM);
 	bpf_map_init_from_attr(&smap->map, attr);
 
+	nbuckets = roundup_pow_of_two(num_possible_cpus());
 	/* Use at least 2 buckets, select_bucket() is undefined behavior with 1 bucket */
-	smap->bucket_log = max_t(u32, 1, ilog2(roundup_pow_of_two(num_possible_cpus())));
-	nbuckets = 1U << smap->bucket_log;
+	nbuckets = max_t(u32, 2, nbuckets);
+	smap->bucket_log = ilog2(nbuckets);
 	cost = sizeof(*smap->buckets) * nbuckets + sizeof(*smap);
 
 	ret = bpf_map_charge_init(&smap->map.memory, cost);