selftests/vm: use memfd for hugepage-mremap test

For dropping the hugetlb mountpoint in run_vmtests.sh. Cleaned it up a
little bit around the changed codes.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Peter Xu <[e-mail]>
Cc: Axel Rasmussen <[e-mail]>
Cc: Mike Kravetz <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/selftests/vm/hugepage-mremap.c

···
 #include <sys/syscall.h> /* Definition of SYS_* constants */
 #include <linux/userfaultfd.h>
 #include <sys/ioctl.h>
+#include <string.h>
 
 #define DEFAULT_LENGTH_MB 10UL
 #define MB_TO_BYTES(x) (x * 1024 * 1024)
···
 int main(int argc, char *argv[])
 {
 	size_t length = 0;
+	int ret = 0, fd;
 
-	if (argc != 2 && argc != 3) {
-		printf("Usage: %s [length_in_MB] <hugetlb_file>\n", argv[0]);
+	if (argc >= 2 && !strcmp(argv[1], "-h")) {
+		printf("Usage: %s [length_in_MB]\n", argv[0]);
 		exit(1);
 	}
 
 	/* Read memory length as the first arg if valid, otherwise fallback to
 	 * the default length.
 	 */
-	if (argc == 3)
-		length = argc > 2 ? (size_t)atoi(argv[1]) : 0UL;
+	if (argc >= 2)
+		length = (size_t)atoi(argv[1]);
+	else
+		length = DEFAULT_LENGTH_MB;
 
-	length = length > 0 ? length : DEFAULT_LENGTH_MB;
 	length = MB_TO_BYTES(length);
-
-	int ret = 0;
-
-	/* last arg is the hugetlb file name */
-	int fd = open(argv[argc-1], O_CREAT | O_RDWR, 0755);
-
+	fd = memfd_create(argv[0], MFD_HUGETLB);
 	if (fd < 0) {
 		perror("Open failed");
 		exit(1);
···
 	}
 
 	close(fd);
-	unlink(argv[argc-1]);
 
 	return ret;
 }