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/[e-mail]
Fixes: a05ef00c9790 ("selftests/vm: add script helper for CONFIG_TEST_VMALLOC_MODULE")
Signed-off-by: Anders Roxell <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: "Uladzislau Rezki (Sony)" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

tools/testing/selftests/vm/config

···
 CONFIG_SYSVIPC=y
 CONFIG_USERFAULTFD=y
+CONFIG_TEST_VMALLOC=m