nfs: provide pnfs_report_layoutstat when NFS42 is disabled

kbuild test robot reported:
   fs/built-in.o: In function `pnfs_report_layoutstat':
>> (.text+0x151a1c): undefined reference to `nfs42_proc_layoutstats_generic'

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/fs/nfs/pnfs.h b/fs/nfs/pnfs.h
index 0a47239..3e6ab7b 100644
--- a/fs/nfs/pnfs.h
+++ b/fs/nfs/pnfs.h
@@ -292,8 +292,6 @@
 struct nfs4_threshold *pnfs_mdsthreshold_alloc(void);
 void pnfs_error_mark_layout_for_return(struct inode *inode,
 				       struct pnfs_layout_segment *lseg);
-int pnfs_report_layoutstat(struct inode *inode);
-
 /* nfs4_deviceid_flags */
 enum {
 	NFS_DEVICEID_INVALID = 0,       /* set when MDS clientid recalled */
@@ -692,4 +690,14 @@
 
 #endif /* CONFIG_NFS_V4_1 */
 
+#if IS_ENABLED(CONFIG_NFS_V4_2)
+int pnfs_report_layoutstat(struct inode *inode);
+#else
+static inline int
+pnfs_report_layoutstat(struct inode *inode)
+{
+	return 0;
+}
+#endif
+
 #endif /* FS_NFS_PNFS_H */