copy: handle mergeable extents across fiemap scans

copy: handle mergeable extents across fiemap scans

* extent-scan.h (extent_scan_free): Init the pointer to NULL,
and reset the count to 0, so that we can realloc the buffer.
* src/extent-scan.c (extent_scan_init): Likewise.
(extent_scan_read): Loop over multiple fiemap scans, so we handle
mergeable extents that span across fiemap scan boundaries.  Once
we have enough unique extents, return so as to minimize memory use.