cp: improve hole handling on squashfs

cp: improve hole handling on squashfs

Better fix for problem reported by Jeremy Allison
<https://bugs.gnu.org/79267>.
* src/copy.c (struct scan_inference): New type, replacing
union scan_inference.  All uses changed.  This is so
infer_scantype can report the first hole's offset when known.
(lseek_copy): 5th arg is now struct scan_inference const *,
not just off_t.  All uses changed.
(infer_scantype): If SEEK_SET+SEEK_HOLE do not find a hole,
fall back on ZERO_SCANTYPE.