Use writev in libfio_sec_osd

rw_blocking.h

@@ -1,6 +1,8 @@
 #pragma once
 
 #include <unistd.h>
+#include <sys/uio.h>
 
 int read_blocking(int fd, void *read_buf, size_t remaining);
 int write_blocking(int fd, void *write_buf, size_t remaining);
+int writev_blocking(int fd, iovec *iov, int iovcnt);