rhashtable: Add cap on number of elements in hash table

We currently have no limit on the number of elements in a hash table.
This is a problem because some users (tipc) set a ceiling on the
maximum table size and when that is reached the hash table may
degenerate.  Others may encounter OOM when growing and if we allow
insertions when that happens the hash table perofrmance may also
suffer.

This patch adds a new paramater insecure_max_entries which becomes
the cap on the table.  If unset it defaults to max_size * 2.  If
it is also zero it means that there is no cap on the number of
elements in the table.  However, the table will grow whenever the
utilisation hits 100% and if that growth fails, you will get ENOMEM
on insertion.

As allowing oversubscription is potentially dangerous, the name
contains the word insecure.

Note that the cap is not a hard limit.  This is done for performance
reasons as enforcing a hard limit will result in use of atomic ops
that are heavier than the ones we currently use.

The reasoning is that we're only guarding against a gross over-
subscription of the table, rather than a small breach of the limit.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>