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>

fs/nfsd/nfs4callback.c

@@ -491,8 +491,6 @@ out_put_cred:
 	 * or deleg_return.
 	 */
 	put_nfs4_client(clp);
-	nfs4_lock_state();
 	nfs4_put_delegation(dp);
-	nfs4_unlock_state();
 	return;
 }