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 <hu.yadi@h3c.com>
Link: https://lore.kernel.org/r/20240112074059.29673-1-hu.yadi@h3c.comAcked-by: Andrei Vagin <avagin@google.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

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

@@ -10,7 +10,6 @@
 #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>
@@ -32,7 +31,11 @@ static int sys_fsmount(int fd, unsigned int flags, unsigned int attr_flags)
 {
 	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)
@@ -166,8 +169,7 @@ int main(int argc, char **argv)
 		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;
 	}