nfsd4: remove use of mutex for file_hashtable

As part of reducing the scope of the client_mutex, and in order to
remove the need for mutexes from the callback code (so that callbacks
can be done as asynchronous rpc calls), move manipulations of the
file_hashtable under the recall_lock.

Update the relevant comments while we're here.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Cc: Alexandros Batsakis <batsakis@netapp.com>
Reviewed-by: Benny Halevy <bhalevy@panasas.com>

include/linux/nfsd/state.h

@@ -214,7 +214,7 @@ struct nfs4_stateowner {
 *      share_acces, share_deny on the file.
 */
 struct nfs4_file {
-	struct kref		fi_ref;
+	atomic_t		fi_ref;
 	struct list_head        fi_hash;    /* hash by "struct inode *" */
 	struct list_head        fi_stateids;
 	struct list_head	fi_delegations;