btrfs: tests: properly initialize fs_info of extent buffer

The fs_info is supposed to be valid, even though it's not used right
now and the test does not crash.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/tests/extent-io-tests.c

@@ -417,7 +417,7 @@ static int test_eb_bitmaps(u32 sectorsize, u32 nodesize)
 
 	/* Do it over again with an extent buffer which isn't page-aligned. */
 	free_extent_buffer(eb);
-	eb = __alloc_dummy_extent_buffer(NULL, nodesize / 2, len);
+	eb = __alloc_dummy_extent_buffer(fs_info, nodesize / 2, len);
 	if (!eb) {
 		test_std_err(TEST_ALLOC_ROOT);
 		ret = -ENOMEM;