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.com
Acked-by: Andrei Vagin <avagin@google.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

selftests/overlayfs: verify device and inode numbers in /proc/pid/maps

When mapping a file on overlayfs, the file stored in ->vm_file is a
backing file whose f_inode is on the underlying filesystem. We need to
verify that /proc/pid/maps contains numbers of the overlayfs file, but
not its backing file.

Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Alexander Mikhalitsyn <alexander@mihalicyn.com>
Signed-off-by: Andrei Vagin <avagin@google.com>
Link: https://lore.kernel.org/r/20231214064439.1023011-2-avagin@google.com
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>