This is a nontrivial change to the NFS client.

Synchronous READ operations are currently done via the ->read() nfs_rpc_op.
Therefore, the synchronous READ path can easily be adapted for NFSv4.  On
the other hand, the asynchronous READ path contains several NFSv3-specific
features, which make it difficult to adapt for NFSv4.

In this patch and the next, we modify the async READ path to be
version-agnostic.  This patch just changes the 'struct nfs_read_data'
so that the v2- and v3-specific parts are moved into a private area,
with room for a v4-specific part in parallel.  None of the logic is
changed.