NFS: Convert v4 into a module

This patch exports symbols needed by the v4 module.  In addition, I also
switch over to using IS_ENABLED() to check if CONFIG_NFS_V4 or
CONFIG_NFS_V4_MODULE are set.

The module (nfs4.ko) will be created in the same directory as nfs.ko and
will be automatically loaded the first time you try to mount over NFS v4.

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/include/linux/nfs_fs.h b/include/linux/nfs_fs.h
index 4b6043c..2889877 100644
--- a/include/linux/nfs_fs.h
+++ b/include/linux/nfs_fs.h
@@ -191,7 +191,7 @@
 	struct hlist_head	silly_list;
 	wait_queue_head_t	waitqueue;
 
-#ifdef CONFIG_NFS_V4
+#if IS_ENABLED(CONFIG_NFS_V4)
 	struct nfs4_cached_acl	*nfs4_acl;
         /* NFSv4 state */
 	struct list_head	open_states;
@@ -428,7 +428,7 @@
  * linux/fs/nfs/file.c
  */
 extern const struct file_operations nfs_file_operations;
-#ifdef CONFIG_NFS_V4
+#if IS_ENABLED(CONFIG_NFS_V4)
 extern const struct file_operations nfs4_file_operations;
 #endif /* CONFIG_NFS_V4 */
 extern const struct address_space_operations nfs_file_aops;
@@ -538,7 +538,7 @@
 extern int nfs_wb_all(struct inode *inode);
 extern int nfs_wb_page(struct inode *inode, struct page* page);
 extern int nfs_wb_page_cancel(struct inode *inode, struct page* page);
-#if defined(CONFIG_NFS_V3) || defined(CONFIG_NFS_V4)
+#if IS_ENABLED(CONFIG_NFS_V3) || IS_ENABLED(CONFIG_NFS_V4)
 extern int  nfs_commit_inode(struct inode *, int);
 extern struct nfs_commit_data *nfs_commitdata_alloc(void);
 extern void nfs_commit_free(struct nfs_commit_data *data);