fd-util: rename loop_get_diskseq() -> fd_get_diskseq()

And move it from loop-util.[ch] -> fd-util.[ch]

src/basic/fd-util.h

@@ -109,6 +109,7 @@ static inline int make_null_stdio(void) {
 int fd_reopen(int fd, int flags);
 int read_nr_open(void);
 int btrfs_defrag_fd(int fd);
+int fd_get_diskseq(int fd, uint64_t *ret);
 
 /* The maximum length a buffer for a /proc/self/fd/<fd> path needs */
 #define PROC_FD_PATH_MAX \