Use a much bigger block size when calling fwrite(). Each call to fwrite()

Use a much bigger block size when calling fwrite().  Each call to fwrite()
writes a few (often around 4) blocks of (I think) directory data to the
drive, so if you only fwrite() one block it ends up writing 1 block of
"payload" and 4 blocks of "admin".  Using bigger blocks makes it faster;
an even bigger block than this makes it a little faster still, but not
significantly.

The extra block writing is seemingly triggered by fwrite() calling
ext4_block_cache_write_back(..., 0). A better fix is probably to call
this another way, at the end of the file copy.