nfsd: Fixes for nfsd4_encode_read_plus_data()

Ensure that we encode the data payload + padding, and that we truncate
the preallocated buffer to the actual read size.

Fixes: 528b8493 ("NFSD: Add READ_PLUS data support")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>