filemap: check if dest is written for every block

filemap: check if dest is written for every block

If lenght parameter is provided to sparse_copy call
it's mandatory to check if the output file is fully
written after reading unmapped block from input file.

If it's not done then sparse_copy can write more data
than specified length.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>