cp: fix copying a sparse file to a pipe

cp: fix copying a sparse file to a pipe

The recent FIEMAP-related changes made it so the unusual case of
copying a sparse file to a non-regular destination (e.g., a pipe)
would erroneously write one byte too many to that destination.
That happened because extent_copy assumed that it could use lseek
to obtain the number of bytes written to the output file descriptor.
That was valid only for regular files.
* src/copy.c (sparse_copy): Add a parameter, to be used by extent_copy,
but not by reg_copy.  Adjust callers.
(extent_copy): Maintain new local, dest_pos, using new arg, n_read.
Don't call lseek on dest_fd; use new var, dest_pos, instead.
(copy_reg): Add unused arg.