u-a: Flush the alternatives database file stream before fsync()ing it

For stream I/O the buffers have to be flushed before they are fsync()ed
to guarantee the data has been written, and as such fsync() can do its
job.

Missed in commit 0e0e26cb8e9e3b8e3a60c329d8ea0c6e5445742c.

utils/update-alternatives.c

@@ -1360,6 +1360,8 @@ alternative_save(struct alternative *a)
 	altdb_print_line(&ctx, "");
 
 	/* Close database file */
+	if (fflush(ctx.fh))
+		syserr(_("unable to flush file '%s'"), ctx.filename);
 	if (fsync(fileno(ctx.fh)))
 		syserr(_("unable to sync file '%s'"), ctx.filename);
 	if (fclose(ctx.fh))