NFSv4.x: Add kernel parameter to control the callback server

Add support for the kernel parameter nfs.callback_nr_threads to set
the number of threads that will be assigned to the callback channel.

Add support for the kernel parameter nfs.nfs.max_session_cb_slots
to set the maximum size of the callback channel slot table.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
diff --git a/fs/nfs/callback.h b/fs/nfs/callback.h
index 14bd863..41ad651 100644
--- a/fs/nfs/callback.h
+++ b/fs/nfs/callback.h
@@ -198,8 +198,9 @@
 #define NFS41_BC_MIN_CALLBACKS 1
 #define NFS41_BC_MAX_CALLBACKS 1
 
-#define NFS4_NR_CALLBACK_THREADS 1
+#define NFS4_MIN_NR_CALLBACK_THREADS 1
 
 extern unsigned int nfs_callback_set_tcpport;
+extern unsigned short nfs_callback_nr_threads;
 
 #endif /* __LINUX_FS_NFS_CALLBACK_H */