Use image file device/inode to find the correct loop device in kpartx

Use image file device/inode to find the correct loop device in kpartx

Previously kpartx used the "lo_name" field of struct loop_info to store
and match the image file name. That field is not intended to store path
names (it's not big enough). Therefore kpartx was unable to delete
mappings to file paths longer than 63 characters. It also didn't
properly handle relative file paths: two different files with identical
relative path names could not be mapped. Instead kpartx would modify
the existing mapping when seeing a new file with the same relative path.

The "loopinfo" structure contains the image file device and inode
numbers. Use those to uniquely identify the correct loop device.