NFS: define and initialize compound_hdr.replen

replen holds the running count of expected reply bytes.
repl will then be used by encoding routines for xdr_inline_pages offset
after which data bytes are to be received directly into the xdr
buffer pages.

NOTE: According to the nfsv4 and v4.1 RFCs, the replied tag SHOULD be the same
is the one sent, but this is not required as a MUST for the server to do so.
The server may screw us if it replies a tag of a different length in the
compound result.

[NFS: cb_compoundhdr.replen is in words not bytes]
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>