Add buffer to i/o batch, with deallocation
#include <iob.h>
int iob_addbuf_free(io_batch* b,const void* buf,uint64 n);
iob_addbuf_free schedules n bytes starting at buf to be written with the I/O batch b.
The buffer will automatically be deallocated by iob_reset.
Returns 1 if successful, 0 if not. In the failure case, the buffer was deallocated.