partitions: Avoid duplicate UMS mounts

If a device has several LUN but only one storage volume TWRP
recognizes (e.g. lun0 for microsd and lun1 for emulated cdrom), then
avoid mounting the same partition twice by checking the mount point.

Change-Id: Ie23b88acca1818873792e9f65b73b125a15e4a5d
1 file changed