stat-util: add vfs_free_bytes()

The casts and the right fields to multiply aren't entirely trivial,
let's add a helper for it.

src/basic/stat-util.h

@@ -161,3 +161,5 @@ static inline bool inode_type_can_hardlink(mode_t m) {
          * type). */
         return IN_SET(m & S_IFMT, S_IFSOCK, S_IFLNK, S_IFREG, S_IFBLK, S_IFCHR, S_IFIFO);
 }
+
+int vfs_free_bytes(int fd, uint64_t *ret);