nfs: Subsequent READDIR calls should carry non-zero cookieverifier

If the loop in nfs_readdir_xdr_to_array() runs more than once, subsequent
READDIR RPCs may wrongly carry a zero cookie verifier and non-zero cookie.
Make sure subsequent calls to READDIR carry the cookie verifier returned
by the first call.

Signed-off-by: Nagendra S Tomar <natomar@microsoft.com>
Fixes: b593c09f ("NFS: Improve handling of directory verifiers")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>