selftests: clarify common error when running gup_test

The gup_test binary will fail showing only the output of perror("open") in
the case that /sys/kernel/debug/gup_test is not found. This will almost
always be due to CONFIG_GUP_TEST not being set, which enables
compilation of a kernel that provides this file.

Add a short error message to clarify this failure and point the user to
the solution.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Joel Savitz <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Nico Pache <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

tools/testing/selftests/vm/gup_test.c

@@ -21,6 +21,8 @@
 #define FOLL_WRITE	0x01	/* check pte is writable */
 #define FOLL_TOUCH	0x02	/* mark page accessed */
 
+#define GUP_TEST_FILE "/sys/kernel/debug/gup_test"
+
 static unsigned long cmd = GUP_FAST_BENCHMARK;
 static int gup_fd, repeats = 1;
 static unsigned long size = 128 * MB;