nfs_common: track all open nfsd_files per LOCALIO nfs_client

nfs_common: track all open nfsd_files per LOCALIO nfs_client

This tracking enables __nfsd_file_cache_purge() to call
nfs_localio_invalidate_clients(), upon shutdown or export change, to
nfs_close_local_fh() all open nfsd_files that are still cached by the
LOCALIO nfs clients associated with nfsd_net that is being shutdown.

Now that the client must track all open nfsd_files there was more work
than necessary being done with the global nfs_uuids_lock contended.
This manifested in various RCU issues, e.g.:
 hrtimer: interrupt took 47969440 ns
 rcu: INFO: rcu_sched detected stalls on CPUs/tasks:

Use nfs_uuid->lock to protect all nfs_uuid_t members, instead of
nfs_uuids_lock, once nfs_uuid_is_local() adds the client to
nn->local_clients.

Also add 'local_clients_lock' to 'struct nfsd_net' to protect
nn->local_clients.  And store a pointer to spinlock in the 'list_lock'
member of nfs_uuid_t so nfs_localio_disable_client() can use it to
avoid taking the global nfs_uuids_lock.

In combination, these split out locks eliminate the use of the single
nfslocalio.c global nfs_uuids_lock in the IO paths (open and close).

Also refactored associated fs/nfs_common/nfslocalio.c methods' locking
to reduce work performed with spinlocks held in general.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>