vm/test_vmalloc.sh: adapt for updated driver interface

A 'single_cpu_test' parameter is odd and it does not exist anymore.
Instead there was introduced a 'nr_threads' one.  If it is not set it
behaves as the former parameter.

That is why update a "stress mode" according to this change specifying
number of workers which are equal to number of CPUs.  Also update an
output of help message based on a new interface.

Link: https://lkml.kernel.org/r/20210402202237.20334-3-urezki@gmail.com
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>