fstab-util: clean up fstab_filter_options

fstab-util: clean up fstab_filter_options

Let's get rid of the confusing goto so that the flow is more
straightforward. Note that the behavior is slightly changed:
previously, ret_filtered would be an empty string even if
the original opts passed in is NULL, but after this commit
it returns NULL too. But this shouldn't matter, as all our
code handles NULL opts gracefully.