Add file region to i/o batch
#include <iob.h>
int iob_addfile(io_batch* b,int64 fd,uint64 off,uint64 n);
iob_addfile schedules n bytes from the file descriptor fd starting at offset off to be written with the I/O batch b.
You may not close fd before iob_send indicates it is done.
Use iob_addfile_close if you want the file descriptor to be closed automatically by iob_reset.
Returns 1 on success, 0 on failure.