mountpoint-util: add public helper for comparing file handles

We already have the code, let's move it to a function of its own and
export it.

src/basic/mountpoint-util.h

@@ -40,6 +40,8 @@ bool is_name_to_handle_at_fatal_error(int err);
 
 int name_to_handle_at_loop(int fd, const char *path, struct file_handle **ret_handle, int *ret_mnt_id, int flags);
 
+bool file_handle_equal(const struct file_handle *a, const struct file_handle *b);
+
 int path_get_mnt_id_at_fallback(int dir_fd, const char *path, int *ret);
 int path_get_mnt_id_at(int dir_fd, const char *path, int *ret);
 static inline int path_get_mnt_id(const char *path, int *ret) {