rhashtable: don't attempt to grow when at max_size

The conversion of mac80211's station table to rhashtable had a bug
that I found by accident in code review, that hadn't been found as
rhashtable apparently managed to have a maximum hash chain length
of one (!) in all our testing.

In order to test the bug and verify the fix I set my rhashtable's
max_size very low (4) in order to force getting hash collisions.

At that point, rhashtable WARNed in rhashtable_insert_rehash() but
didn't actually reject the hash table insertion. This caused it to
lose insertions - my master list of stations would have 9 entries,
but the rhashtable only had 5. This may warrant a deeper look, but
that WARN_ON() just shouldn't happen.

Fix this by not returning true from rht_grow_above_100() when the
rhashtable's max_size has been reached - in this case the user is
explicitly configuring it to be at most that big, so even if it's
now above 100% it shouldn't attempt to resize.

This fixes the "lost insertion" issue and consequently allows my
code to display its error (and verify my fix for it.)

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/rhashtable.h

@@ -282,7 +282,8 @@ static inline bool rht_shrink_below_30(const struct rhashtable *ht,
 static inline bool rht_grow_above_100(const struct rhashtable *ht,
 				      const struct bucket_table *tbl)
 {
-	return atomic_read(&ht->nelems) > tbl->size;
+	return atomic_read(&ht->nelems) > tbl->size &&
+		(!ht->p.max_size || tbl->size < ht->p.max_size);
 }
 
 /* The bucket lock is selected based on the hash and protects mutations