rcutorture: Add nwriters module parameter

Believe it or not, there are people who would like to run rcutorture
without actually torturing RCU.  For example, some people would like to
induce various types of stall warnings without placing any unnecessary
additional overhead on their systems running in production.  And
rcutorture provides the stall_cpu, stall_cpu_holdoff, stall_no_softlockup,
stall_cpu_irqsoff, stall_cpu_block, and stall_cpu_repeat module parameters
in order to allow the user to force numerous types of stalls.  In addition,
rcutorture provides a great number of other module parameters to allow the
user to reduce other overhead.

But unfortunately, there is no way to turn of the rcu_torture_writer()
portion of this torture test, which on my x86 laptop consumes somewhere
between 40% and 45% of a CPU.  Although this is quite lightweight for a
torture test, it is not welcome on systems running production workloads.
This commit therefore adds an nwriters module parameter that defaults
to 1 but can be set to 0 in order to disable the rcu_torture_writer()
portion of the torture test, but that cannot be set to any other value
(that is what the fakewriters module parameter is for!).  This reduces
the overhead to well under 1% of a CPU, which is much more likely to be
compatible with production workloads.

Reported-by: Breno Leitao <leitao@debian.org>
Reported-by: Puranjay Mohan <puranjay@kernel.org>
Reported-by: Usama Arif <usama.arif@linux.dev>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>