use /dev/mapper name for LVM devices Fixes #10257

LVM devices can be referenced by either
`/dev/vgname/lvname` or `/dev/mapper/vgname-lvname`
However mount always returns the `/dev/mapper` name.
We should convert names to the `/dev/mapper` format.
Merge pull request #10570 from ahammond/normalize_mapper_mounts

guess at how to solve issue #10257