shmem: fix recovery on rename failures

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

shmem: fix recovery on rename failures

maple_tree insertions can fail if we are seriously short on memory; simple_offset_rename() does not recover well if it runs into that. The same goes for simple_offset_rename_exchange().

Moreover, shmem_whiteout() expects that if it succeeds, the caller will progress to d_move(), i.e. that shmem_rename2() won't fail past the successful call of shmem_whiteout().

Not hard to fix, fortunately - mtree_store() can't fail if the index we are trying to store into is already present in the tree as a singleton.

For simple_offset_rename_exchange() that's enough - we just need to be careful about the order of operations.

For simple_offset_rename() solution is to preinsert the target into the tree for new_dir; the rest can be done without any potentially failing operations.

That preinsertion has to be done in shmem_rename2() rather than in simple_offset_rename() itself - otherwise we'd need to deal with the possibility of failure after successful shmem_whiteout().