nfsd: convert global state_lock to per-net deleg_lock

nfsd: convert global state_lock to per-net deleg_lock

Replace the global state_lock spinlock with a per-nfsd_net deleg_lock.
The state_lock was only used to protect delegation lifecycle operations
(the del_recall_lru list and delegation hash/unhash), all of which are
scoped to a single network namespace. Making the lock per-net removes
a source of unnecessary contention between containers.

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