Ondrej Holy authored Jul 30, 2018 and Philip Withnall committed Aug 03, 2018

The documentation claims that g_volume_get_mount should	succeed	after
g_volume_mount. Let's update mounts before releasing g_volume_mount to
be sure that the mount is added to the corresponding volume. The same
is done in GVfsUDisks2VolumeMonitor.

(Backported from commit 9b6b282e with no
merge conflicts.)

#1458