proc: return exit code 4 for skipped tests

Test harness uses 4 for SKIP, not 2.

Link: http://lkml.kernel.org/r/20190108193108.GA12259@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/proc/proc-self-map-files-002.c

@@ -63,7 +63,7 @@ int main(void)
 	p = mmap((void *)va, PAGE_SIZE, PROT_NONE, MAP_PRIVATE|MAP_FILE|MAP_FIXED, fd, 0);
 	if (p == MAP_FAILED) {
 		if (errno == EPERM)
-			return 2;
+			return 4;
 		return 1;
 	}