CVE-2023-53606: nfsd: clean up potential nfsd_file refcount leaks in COPY codepath

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

nfsd: clean up potential nfsdfile refcount leaks in COPY codepath

There are two different flavors of the nfsd4copy struct. One is embedded in the compound and is used directly in synchronous copies. The other is dynamically allocated, refcounted and tracked in the client struture. For the embedded one, the cleanup just involves releasing any nfsdfiles held on its behalf. For the async one, the cleanup is a bit more involved, and we need to dequeue it from lists, unhash it, etc.

There is at least one potential refcount leak in this code now. If the kthreadcreate call fails, then both the src and dst nfsdfiles in the original nfsd4copy object are leaked.

The cleanup in this codepath is also sort of weird. In the async copy case, we'll have up to four nfsdfile references (src and dst for both flavors of copy structure). They are both put at the end of nfsd4doasynccopy, even though the ones held on behalf of the embedded one outlive that structure.

Change it so that we always clean up the nfsdfile refs held by the embedded copy structure before nfsd4copy returns. Rework cleanupasynccopy to handle both inter and intra copies. Eliminate nfsd4cleanupintrassc since it now becomes a no-op.