um: ubd: Add REQ_FLUSH suppport

UML's block device driver does not support write barriers,
to support this this patch adds REQ_FLUSH suppport.
Every time the block layer sends a REQ_FLUSH we fsync() now
our backing file to guarantee data consistency.

Reported-and-tested-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/os-Linux/file.c

@@ -266,6 +266,15 @@ int os_write_file(int fd, const void *buf, int len)
 	return n;
 }
 
+int os_sync_file(int fd)
+{
+	int n = fsync(fd);
+
+	if (n < 0)
+		return -errno;
+	return n;
+}
+
 int os_file_size(const char *file, unsigned long long *size_out)
 {
 	struct uml_stat buf;