lockd: Move xdr4.h from include/linux/lockd/ to fs/lockd/

lockd: Move xdr4.h from include/linux/lockd/ to fs/lockd/

The xdr4.h header declares NLMv4-specific XDR encoder/decoder
functions and error codes that are used exclusively within the
lockd subsystem. Moving it from include/linux/lockd/ to fs/lockd/
clarifies the intended scope of these declarations and prevents
external code from depending on lockd-internal interfaces.

This change reduces the public API surface of the lockd module
and makes it easier to refactor NLMv4 internals without risk of
breaking out-of-tree consumers. The header's contents are
implementation details of the NLMv4 wire protocol handling, not
a contract with other kernel subsystems.

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