To avoid various livelocks, the writeback code parks all the dirty inodes onto
sb->s_io and then works through that list until it is empty.  This assumes
that each inode will be moved to some other list as it is processed.

But there's a loophole: if the ->writepages() implementation does nothing at
all, the inode is not redirtied (which would move it to s_dirty).  This causes
s_io to not empty and pdflush goes nuts.

So when this happens, move the inode onto s_dirty within
__sync_single_inode().  Use list_move_tail() to attempt to preserve the
time-ordering of the s_dirty list.