Unshare ums when SD card is removed

If SD card is removed without unmounting when in USB mass storage mode
the ums share is not properly closed and the device files 179:0 and 179:1
are left in use. This causes erratic behaviour on subsequent operations on
the card, i.e. mounting and formatting will fail.

Change-Id: I757703c6282f4b76e2d8b027f4644920737309b6
1 file changed