copy: only check for traversing mount points on directories

copy: only check for traversing mount points on directories

This fixes the copy routines on overlay filesystem, which typically
returns the underlying st_dev for files, symlinks, etc.

The value of st_dev is guaranteed to be the same for directories, so
checking it on directories only fixes this code on overlay filesystem
and still keeps it from traversing mount points (which was the original
intent.)

There's a small side effect here, by which regular (non-directory) files
with bind mounts will be copied by the new logic (while they were
skipped by the previous logic.)

Tested: ./build/test-copy with an overlay on /tmp.

Fixes: #9134