cp: --sparse=always was missing some holes

cp: --sparse=always was missing some holes

The sparse code assumed that st_blksize was the minimum hole size.
However, st_blksize is an optimum I/O buffer size, not the file
system fundamental block size.  Use ST_NBLOCKSIZE instead;
although it may underestimate the true block size that just slows
‘cp’ down a bit, without introducing bugs.
* src/copy.c (sparse_copy): Arg scan_holes replaces
the old hole_size arg.  All callers changed.
(lseek_copy): Remove hole_size arg; no longer needed.
Caller changed.