file: unionize lfsr/randommap

We only use one of them, add file flags to distinguish between the
two.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/io_u.c b/io_u.c
index efbcea9..23a9e4a 100644
--- a/io_u.c
+++ b/io_u.c
@@ -100,6 +100,8 @@
 	} else {
 		uint64_t off = 0;
 
+		assert(fio_file_lfsr(f));
+
 		if (lfsr_next(&f->lfsr, &off))
 			return 1;