nfsd: split nfsd4_copy into transient and durable async copy objects

struct nfsd4_copy served two roles: as &u->copy it is a transient
per-COMPOUND argument in the request buffer; as the heap async_copy it is
a durable object (worker kthread, reaper linkage, CB_OFFLOAD callback, IDR
stateid) that outlives the COMPOUND, with dup_copy_fields() shuttling
state between them. That dual identity was the root of the recent lifetime
bugs.

Introduce struct nfsd4_async_copy for the durable object. It embeds a
struct nfsd4_copy (cp_copy) for the operation parameters/result and adds
the durable-only fields: async_copies linkage, task_struct, refcount,
reaper TTL, copy stateid, and CB_OFFLOAD callback. The durable object
therefore never points into the request buffer. cp_clp stays in
nfsd4_copy -- it is a request property read by the sync-copy tracepoints
on the transient object.

Mechanical split, no intended behavioral change; a step toward folding the
copy stateids into the common nfs4_stid infrastructure.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260710-nfsd-testing-v3-8-a0ff7db6aa3e@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

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