lockd: Make linux/lockd/nlm.h an internal header

lockd: Make linux/lockd/nlm.h an internal header

The NLM protocol constants and status codes in nlm.h are needed
only by lockd's internal implementation. NFS client code and
NFSD interact with lockd through the stable API in bind.h and
have no direct use for protocol-level definitions.

Exposing these definitions globally via bind.h creates unnecessary
coupling between lockd internals and its consumers. Moving nlm.h
from include/linux/lockd/ to fs/lockd/ clarifies the API boundary:
bind.h provides the lockd service interface, while nlm.h remains
available only to code within fs/lockd/ that implements the
protocol.

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