NFSv4.1: Convert pNFS deviceid to use kfree_rcu()

Use of synchronize_rcu() when unmounting and potentially freeing a lot
of deviceids is problematic. There really is no reason why we can't just
use kfree_rcu() here.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/pnfs.h

@@ -302,6 +302,7 @@ struct nfs4_deviceid_node {
 	unsigned long 			flags;
 	unsigned long			timestamp_unavailable;
 	struct nfs4_deviceid		deviceid;
+	struct rcu_head			rcu;
 	atomic_t			ref;
 };