Add buffer to i/o batch
#include <iob.h>
int iob_addbuf(io_batch* b,const void* buf,uint64 n);
iob_addbuf schedules n bytes starting at buf to be written with the I/O batch b.
Use iob_addbuf_free if you want the buffer to be deallocated automatically by iob_reset.
Returns 1 on success, 0 on failure.