Add a test for comparing the performance of RCU with various read-side
synchronization mechanisms. The test has proved useful for collecting
data and performing these comparisons.

Currently RCU, SRCU, reader-writer lock, reader-writer semaphore and
reference counting can be measured using refperf.perf_type parameter.
Each invocation of the test runs measures performance of a specific
mechanism.

The maximum number of CPUs to concurrently run readers on is chosen by
the test itself and is 75% of the total number of CPUs. So if you had 24
CPUs, the test runs with a maximum of 18 parallel readers.

A number of experiments are conducted, and in each experiment, the
number of readers is increased by 1, upto the 75% of CPUs mark. During
each experiment, all readers execute an empty loop with refperf.loops
iterations and time the total loop duration. This is then averaged.

Example output:
Parameters "refperf.perf_type=srcu refperf.loops=2000000" looks like:

[    3.347133] srcu-ref-perf:
[    3.347133] Threads  Time(ns)
[    3.347133] 1        36
[    3.347133] 2        34
[    3.347133] 3        34
[    3.347133] 4        34
[    3.347133] 5        33
[    3.347133] 6        33
[    3.347133] 7        33
[    3.347133] 8        33
[    3.347133] 9        33
[    3.347133] 10       33
[    3.347133] 11       33
[    3.347133] 12       33
[    3.347133] 13       33
[    3.347133] 14       33
[    3.347133] 15       32
[    3.347133] 16       33
[    3.347133] 17       33
[    3.347133] 18       34
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>