NFSD: Add nfsd4_encode_lock_owner4()

To improve readability and better align the LOCK encoders with the
XDR specification, add an explicit encoder named for the lock_owner4
type.

In particular, to avoid code duplication, use
nfsd4_encode_clientid4() to encode the clientid in the lock owner
rather than open-coding it.

It looks to me like nfs4_set_lock_denied() already clears the
clientid if it won't return an owner (cf: the nevermind: label). The
code in the XDR encoder appears to be redundant and can safely be
removed.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>