lib: tst_mkfs: Allow passing more extra options

This commit changes the extra_opt pointer from a string to an array so
that we can pass more than one extra option to the mkfs and also adjusts
two tests that are using this feature.

+ Fixes the docs.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
diff --git a/lib/tst_test.c b/lib/tst_test.c
index cedab53..1280400 100644
--- a/lib/tst_test.c
+++ b/lib/tst_test.c
@@ -745,7 +745,7 @@
 {
 	if (tst_test->format_device) {
 		SAFE_MKFS(tdev.dev, tdev.fs_type, tst_test->dev_fs_opts,
-			  tst_test->dev_extra_opt);
+			  tst_test->dev_extra_opts);
 	}
 
 	if (tst_test->needs_rofs) {