NFS: Clean up: revert increase in READDIR RPC buffer max size

Security labels go with each directory entry, thus they are always
stored in the page cache, not in the head buffer.  The length of the
reply that goes in head[0] should not have changed to support
NFSv4.2 labels.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>