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/shared/btrfs-util.c

@@ -739,6 +739,18 @@ int btrfs_subvol_get_subtree_quota(const char *path, uint64_t subvol_id, BtrfsQu
         return btrfs_subvol_get_subtree_quota_fd(fd, subvol_id, ret);
 }
 
+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 btrfs_defrag(const char *p) {
         _cleanup_close_ int fd = -1;
 

src/shared/btrfs-util.h

@@ -52,6 +52,7 @@ int btrfs_clone_range(int infd, uint64_t in_offset, int ofd, uint64_t out_offset
 int btrfs_get_block_device_fd(int fd, dev_t *dev);
 int btrfs_get_block_device(const char *path, dev_t *dev);
 
+int btrfs_defrag_fd(int fd);
 int btrfs_defrag(const char *p);
 
 int btrfs_quota_enable_fd(int fd, bool b);