qfile-unpacker: do not call fdatasync() at each file (#1257)

POSIX  requires  that  a  read(2)  which  can be proved to occur after a
write() has returned returns the new data.
We want here only that other processes in the same VM will see the
file either fully written, or not see it at all. So ensuring that
linkat(2) is called after write is completed should be enough.

Fixes QubesOS/qubes-issues#1257

qrexec-lib/unpack.c

@@ -137,7 +137,6 @@ void process_one_file_reg(struct file_header *untrusted_hdr,
 		else
 			do_exit(errno, untrusted_name);
 	}
-	fdatasync(fdout);
 	if (use_tmpfile) {
 		char fd_str[7];
 		snprintf(fd_str, sizeof(fd_str), "%d", fdout);