copy: use FIEMAP (extent_copy) only for apparently-sparse files,
to avoid the expense of extent_copy's unconditional use of
FIEMAP_FLAG_SYNC.
* src/copy.c (copy_reg): Do not attempt extent_copy on a file
that appears to have no holes.
* NEWS (Changes in behavior): Document this. At first I labeled this
as a bug fix, but that would be inaccurate, considering there is no
documentation of FIEMAP semantics, nor even consensus among kernel
FS developers. Here's hoping SEEK_HOLE/SEEK_DATA support will soon
make it into the linux kernel.