Infinite loop in nffs_gc()

[yashi]

This is a classic "Do not reorder while traversing a list" bug from the nffs_hash in nffs_gc().

In the while-loop to traverse nffs_hash list in nffs_gc(), we call nffs_gc_inode_blocks(). But nffs_gc_inode_blocks() reorders the given list when GC. If the reorder puts the current node back to previous, then call nffs_gc_inode_blocks() again, it will inf loop.

That is, say we have a list of A->B->C->D. while we are at C, we call nffs_gc_inode_blocks(). The function can reorder the list to A->C->B->D. Now we process B, and call nffs_gc_inode_blocks(). This time the function reorder the list back to to A->B->C->D. The next node is C again. C call the func, and get A->C->B->D. The next is B. Call the func. Get A->B->C->D. Then next is C. And B, and C, B, C, B, C, B, C.... Inf loop.