rcutorture: Use nr_cpus rather than maxcpus to limit test size

rcutorture: Use nr_cpus rather than maxcpus to limit test size

The maxcpus= kernel boot parameter limits the number of CPUs brought
online at boot time, but it does nothing to prevent additional CPUs
from being brought up later.  Placing a hard cap on the total number
of CPUs is instead the job of the nr_cpus= boot parameter.  This commit
therefore switches the configfrag_boot_cpus() shell function from maxcpus=
to nr_cpus=.  This commit also adds a nr_cpus=43 kernel parameter to RCU's
TREE01 test scenario, but retains the maxcpus=8 kernel parameter in order
to test the ability of RCU expedited grace periods to handle new CPUs
coming online for the first time during grace-period initialization.
Finally, this commit makes the torture scheduling allow maxcpus= to
override other means of specifying the number of CPUs to allow for.
This last works because the torture kernel modules size their workloads
based on the number of CPUs present at the start of the test, not the
ultimate number of CPUs.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>