NFS: move credential expiry tracking out of SUNRPC into NFS.

NFS needs to know when a credential is about to expire so that
it can modify write-back behaviour to finish the write inside the
expiry time.
It currently uses functions in SUNRPC code which make use of a
fairly complex callback scheme and flags in the generic credientials.

As I am working to discard the generic credentials, this has to change.

This patch moves the logic into NFS, in part by finding and caching
the low-level credential in the open_context.  We then make direct
cred-api calls on that.

This makes the code much simpler and removes a dependency on generic
rpc credentials.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

include/linux/nfs_fs.h

@@ -71,6 +71,7 @@ struct nfs_open_context {
 	fl_owner_t flock_owner;
 	struct dentry *dentry;
 	struct rpc_cred *cred;
+	struct rpc_cred *ll_cred;	/* low-level cred - use to check for expiry */
 	struct nfs4_state *state;
 	fmode_t mode;