This is a nontrivial change to the NFS client.

In this patch, we finish modifying the async READ path so that it is
version-agnostic.  We define a new nfs_rpc_op ->setup_read(), and move
the v2- and v3-specific code in nfs_read_rpcsetup() there.  We also
have to change nfs_readpage() result so that the 'count' of bytes
read is a parameter.  The extra parameter means that it can no longer
be ->tk_exit().  Instead, it is called from a version-specific ->tk_exit()
routine which is set in ->read_setup().

The upshot of all this is that the version-specific part of the
async READ path has been encapsulated in a new nfs_rpc_op
->read_setup(), and NFSv4 can share the logic for asynchronous
READ's with NFSv2 and v3.