cryptsetup: use more descriptive name for the variable and drop redundant function

cryptsetup: use more descriptive name for the variable and drop redundant function

Let's rename escaped_name to disk_path since this is an actual content
that pointer refers to. It is either path to encrypted block device
or path to encrypted image file.

Also drop redundant function disk_major_minor(). src is always set, and
it always points to either encrypted block device path (or symlink to
such device) or to encrypted image. In case it is set to device path
there is no need to reset it to /dev/block/major:minor symlink since
those paths are equivalent.