jbd2: remove wrong sb->s_sequence check

In the Linux kernel, the following vulnerability has been resolved:

jbd2: remove wrong sb->s_sequence check

Journal emptiness is not determined by sb->s_sequence == 0 but rather by

sb->s_start == 0 (which is set a few lines above). Furthermore 0 is a

valid transaction ID so the check can spuriously trigger. Remove the

invalid WARN_ON.