Add more context to the error messages

Errors like:

fio: pid=0, err=22/file:filesetup.c:380, error=Invalid argument

do not give a lot of clue as to what is wrong, unless you
have a matching source. So add a context relevant info
message as well.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/ioengines.c b/ioengines.c
index 441f36f..0b3ec16 100644
--- a/ioengines.c
+++ b/ioengines.c
@@ -102,7 +102,7 @@
 	dlerror();
 	dlhandle = dlopen(engine_lib, RTLD_LAZY);
 	if (!dlhandle) {
-		td_vmsg(td, -1, dlerror());
+		td_vmsg(td, -1, dlerror(), "dlopen");
 		return NULL;
 	}
 
@@ -112,7 +112,7 @@
 	 */
 	ops = dlsym(dlhandle, "ioengine");
 	if (!ops) {
-		td_vmsg(td, -1, dlerror());
+		td_vmsg(td, -1, dlerror(), "dlsym");
 		dlclose(dlhandle);
 		return NULL;
 	}
@@ -234,6 +234,8 @@
 
 int td_io_commit(struct thread_data *td)
 {
+	if (!td->cur_depth)
+		return 0;
 	if (td->io_ops->commit)
 		return td->io_ops->commit(td);