mount-util: add new path_get_mnt_id() call that queries the mnt ID of a path

This is a simple wrapper around name_to_handle_at_loop() and
fd_fdinfo_mnt_id() to query the mnt ID of a path. It uses
name_to_handle_at() where it can, and falls back to to
fd_fdinfo_mnt_id() where that doesn't work.

This is a best-effort thing of course, since neither name_to_handle_at()
nor the fdinfo logic work on all kernels.

src/basic/mount-util.c

@@ -302,6 +302,16 @@ int path_is_mount_point(const char *t, const char *root, int flags) {
         return fd_is_mount_point(fd, basename(t), flags);
 }
 
+int path_get_mnt_id(const char *path, int *ret) {
+        int r;
+
+        r = name_to_handle_at_loop(AT_FDCWD, path, NULL, ret, 0);
+        if (IN_SET(r, -EOPNOTSUPP, -ENOSYS, -EACCES, -EPERM)) /* kernel/fs don't support this, or seccomp blocks access */
+                return fd_fdinfo_mnt_id(AT_FDCWD, path, 0, ret);
+
+        return r;
+}
+
 int umount_recursive(const char *prefix, int flags) {
         bool again;
         int n = 0, r;

src/basic/mount-util.h

@@ -32,6 +32,8 @@
 
 int name_to_handle_at_loop(int fd, const char *path, struct file_handle **ret_handle, int *ret_mnt_id, int flags);
 
+int path_get_mnt_id(const char *path, int *ret);
+
 int fd_is_mount_point(int fd, const char *filename, int flags);
 int path_is_mount_point(const char *path, const char *root, int flags);