selftests: vm: add fragment CONFIG_GUP_BENCHMARK

When running gup_benchmark test the following output states that
the config options is missing.

$ sudo ./gup_benchmark
open: No such file or directory

$ sudo strace -e trace=file ./gup_benchmark 2>&1 | tail -3
openat(AT_FDCWD, "/sys/kernel/debug/gup_benchmark", O_RDWR) = -1 ENOENT
(No such file or directory)
open: No such file or directory
+++ exited with 1 +++

Fix it by adding config option fragment.

Fixes: 64c349f4 ("mm: add infrastructure for get_user_pages_fast() benchmarking")
Signed-off-by: Anatoly Pugachev <matorola@gmail.com>
CC: Jiri Kosina <trivial@kernel.org>
CC: Shuah Khan <shuah@kernel.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

tools/testing/selftests/vm/config

@@ -3,3 +3,4 @@ CONFIG_USERFAULTFD=y
 CONFIG_TEST_VMALLOC=m
 CONFIG_DEVICE_PRIVATE=y
 CONFIG_TEST_HMM=m
+CONFIG_GUP_BENCHMARK=y