tools/testing/selftests/vm/gup_benchmark.c: fix 'write' flag usage

If the '-w' parameter was provided, the benchmark would exit due to a
mssing 'break'.

Link: http://lkml.kernel.org/r/20181010195605.10689-3-keith.busch@intel.comSigned-off-by: Keith Busch <keith.busch@intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/vm/gup_benchmark.c

@@ -60,6 +60,7 @@ int main(int argc, char **argv)
 			break;
 		case 'w':
 			write = 1;
+			break;
 		default:
 			return -1;
 		}