selftests: vm: add fragment CONFIG_TEST_VMALLOC

When running test_vmalloc.sh smoke the following print out states that
the fragment is missing.

 # ./test_vmalloc.sh: You must have the following enabled in your kernel:
 # CONFIG_TEST_VMALLOC=m

Rework to add the fragment 'CONFIG_TEST_VMALLOC=m' to the config file.

Link: http://lkml.kernel.org/r/20190916095217.19665-1-anders.roxell@linaro.org
Fixes: a05ef00c

 ("selftests/vm: add script helper for CONFIG_TEST_VMALLOC_MODULE")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>