lockd: Introduce nlm__int__deadlock

lockd: Introduce nlm__int__deadlock

The use of CONFIG_LOCKD_V4 in combination with a later cast_status()
in the NLMv3 code is difficult to reason about. Instead, replace the
use of nlm_deadlock with an implementation-defined status value that
version-specific code translates appropriately.

The new approach establishes a translation boundary: generic lockd
code returns nlm__int__deadlock when posix_lock_file() yields
-EDEADLK. Version-specific handlers (svc4proc.c for NLMv4,
svcproc.c for NLMv3) translate this internal status to the
appropriate wire protocol value. NLMv4 maps to nlm4_deadlock;
NLMv3 maps to nlm_lck_denied (since NLMv3 lacks a deadlock-specific
status code).

Later this modification will also remove the need to include NLMv4
headers in NLMv3 and generic code.

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