cp: don’t allocate a separate zero buffer

cp: don’t allocate a separate zero buffer

* src/copy-file-data.c (write_zeros): New args abuf, buf_size.
Use the lazily-allocated buffer, which most likely already exists,
rather than allocating a separate buffer just for zeros.
This makes the code more reentrant as there is no longer
a need for static storage here.  Although there is some CPU
overhead due to the need to zero out the buffer for each file,
the overhead is relatively small as the buffer is smallish
and should be cached.  All callers changed.