In NFSv3, an RPC is retried if the special error NFSERR_JUKEBOX is
received.  This generic bit of postprocessing happens invisibly for
synchronous RPC's, but in the async case, the ->tk_exit callback
must call nfs_async_handle_jukebox() by hand.

In NFSv4, we also need generic postprocessing of async RPC's, but
the details are different.  Therefore, we don't want to call
nfs_async_handle_jukebox(); we want to call a different, NFSv4-specific
routine.  Therefore, we want to move calls to nfs_async_handle_jukebox()
out of the "generic" NFS code and into NFSv3-specific routines.  This
has already been done for async READ and WRITE in the preceding patches,
but there is still one outstanding case: the async REMOVE in sillyrename.

This patch removes nfs_async_handle_jukebox() from the async sillyrename
path, and puts in the NFSv3 ->unlink_done() rpc_op.