lib/test_vmalloc.c: add a new 'nr_threads' parameter

By using this parameter we can specify how many workers are created to
perform vmalloc tests.  By default it is one CPU.  The maximum value is
set to 1024.

As a result of this change a 'single_cpu_test' one becomes obsolete,
therefore it is no longer needed.

[urezki@gmail.com: extend max value of nr_threads parameter]
  Link: https://lkml.kernel.org/r/20210406124536.19658-1-urezki@gmail.com

Link: https://lkml.kernel.org/r/20210402202237.20334-2-urezki@gmail.com
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
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: Shuah Khan <shuah@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>