btrfs: Add a workaround for btrfs failures seen with kernel 3.7.0.

See:
https://bugzilla.redhat.com/show_bug.cgi?id=863978
and:
http://thread.gmane.org/gmane.comp.file-systems.btrfs/20257

daemon/mkfs.c

@@ -192,6 +192,13 @@ do_mkfs (const char *fstype, const char *device, int blocksize,
   }
 
   free (err);
+
+  /* Workaround for RHBZ#863978, possibly in kernel 3.7.0.  Can be
+   * fixed when we work out what's going on upstream.
+   */
+  if (STREQ (fstype, "btrfs"))
+    sync_disks ();
+
   return 0;
 }