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

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

The lockd subsystem unnecessarily exposes internal NLM XDR type
definitions through the global include path. These definitions
are not used by any code outside fs/lockd/, making them
inappropriate for include/linux/lockd/.

Moving xdr.h to fs/lockd/ narrows the API surface and clarifies
that these types are internal implementation details. The
comment in linux/lockd/bind.h stating xdr.h was needed for
"xdr-encoded error codes" is stale: no lockd API consumers use
those codes.

Forward declarations for struct nfs_fh and struct file_lock are
added to bind.h because their definitions were previously pulled
in transitively through xdr.h. Additionally, nfs3proc.c and
proc.c need explicit includes of filelock.h for FL_CLOSE and
for accessing struct file_lock members, respectively.

Built and tested with lockd client/server operations. No
functional change.

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