rhashtable-test: extend to test concurrency

After having tested insertion, lookup, table walk and removal, spawn a
number of threads running operations on the same rhashtable. Each of
them will:

1) insert it's own set of objects,
2) lookup every successfully inserted object and finally
3) remove objects in several rounds until all of them have been removed,
   making sure the remaining ones are still found after each round.

This should put a good amount of load onto the system and due to
synchronising thread startup via two semaphores also extensive
concurrent table access.

The default number of ten threads returned within half a second on my
local VM with two cores. Running 200 threads took about four seconds. If
slow systems suffer too much from this though, the default could be
lowered or even set to zero so this extended test does not run at all by
default.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>