selftests/filesystems:fix build error in overlayfs

One build issue comes up due to both mount.h included dev_in_maps.c

In file included from dev_in_maps.c:10:
/usr/include/sys/mount.h:35:3: error: expected identifier before numeric constant
35 | MS_RDONLY = 1, /* Mount read-only. */
| ^~~~~~~~~
In file included from dev_in_maps.c:13:

Remove one of them to solve conflict, another error comes up:

dev_in_maps.c:170:6: error: implicit declaration of function ‘mount’ [-Werror=implicit-function-declaration]
170 | if (mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL) == -1) {
| ^~~~~
cc1: all warnings being treated as errors

and then , add sys_mount definition to solve it
After both above, dev_in_maps.c can be built correctly on my mache(gcc 10.2,glibc-2.32,kernel-5.10)

Signed-off-by: Hu Yadi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Acked-by: Andrei Vagin <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

tools/testing/selftests/filesystems/overlayfs/dev_in_maps.c

···
 #include <linux/mount.h>
 #include <sys/syscall.h>
 #include <sys/stat.h>
-#include <sys/mount.h>
 #include <sys/mman.h>
 #include <sched.h>
 #include <fcntl.h>
···
 {
 	return syscall(__NR_fsmount, fd, flags, attr_flags);
 }
-
+static int sys_mount(const char *src, const char *tgt, const char *fst,
+		unsigned long flags, const void *data)
+{
+	return syscall(__NR_mount, src, tgt, fst, flags, data);
+}
 static int sys_move_mount(int from_dfd, const char *from_pathname,
 			  int to_dfd, const char *to_pathname,
 			  unsigned int flags)
···
 		ksft_test_result_skip("unable to create a new mount namespace\n");
 		return 1;
 	}
-
-	if (mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL) == -1) {
+	if (sys_mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL) == -1) {
 		pr_perror("mount");
 		return 1;
 	}