g_unix_mounts_get() ignores btrfs subvolumes

This causes some misbehaviors like nautilus or 'gvfs-ls trash://' fails to list
trashed files. (e.g. http://askubuntu.com/questions/537055/ )

I think the culprit is the following portion of _g_get_unix_mounts() from
gio/gunixmounts.c:

|      /* ignore any mnt_fsname that is repeated and begins with a '/'
|       *
|       * We do this to avoid being fooled by --bind mounts, since
|       * these have the same device as the location they bind to.
|       * It's not an ideal solution to the problem, but it's likely that
|       * the most important mountpoint is first and the --bind ones after
|       * that aren't as important. So it should work.
|       *
|       * The '/' is to handle procfs, tmpfs and other no device mounts.
|       */
|      if (mntent->mnt_fsname != NULL &&
|         mntent->mnt_fsname[0] == '/' &&
|         g_hash_table_lookup (mounts_hash, mntent->mnt_fsname))
|        continue;

https://git.gnome.org/browse/glib/tree/gio/gunixmounts.c#n392

The code is not aware of the btrfs subvolumes.