In order to determine whether to trash a file to the home directory, we
compare its st_dev to our home directory's st_dev field.

This is the wrong thing to do on overlayfs when deleting files, because
st_dev contains the ID of the filesystem providing the file (which can
be the lower or upper filesystem), but directories always return the ID
of the overlayfs. Thus the comparison fails and we are unable to trash
the file.

Fix this by checking st_dev of the parent directory when we are deleting
a file.

Also adjust `test_trash_not_supported` for this - make its st_dev check
look at the parent directory's `st_dev` rather than the temporary file's
own.

Fixes #1027.