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.c

@@ -2,7 +2,6 @@
 
 #include <errno.h>
 #include <fcntl.h>
-#include <linux/btrfs.h>
 #if WANT_LINUX_FS_H
 #include <linux/fs.h>
 #endif
@@ -779,20 +778,6 @@ int read_nr_open(void) {
         return 1024 * 1024;
 }
 
-/* This is here because it's fd-related and is called from sd-journal code. Other btrfs-related utilities are
- * in src/shared, but libsystemd must not link to libsystemd-shared, see docs/ARCHITECTURE.md. */
-int btrfs_defrag_fd(int fd) {
-        int r;
-
-        assert(fd >= 0);
-
-        r = fd_verify_regular(fd);
-        if (r < 0)
-                return r;
-
-        return RET_NERRNO(ioctl(fd, BTRFS_IOC_DEFRAG, NULL));
-}
-
 int fd_get_diskseq(int fd, uint64_t *ret) {
         uint64_t diskseq;
 

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 */