btrfs-util: move btrfs_defrag_fd() from fd-util.[ch]

After d71ece3f0b, the function is not used
in libbasic or libsystemd anymore. Let's move it to more appropriate
place.

src/basic/fd-util.h

@@ -108,7 +108,6 @@ 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 */