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>

fs/nfs/pnfs.c

@@ -2252,6 +2252,7 @@ struct nfs4_threshold *pnfs_mdsthreshold_alloc(void)
 	return thp;
 }
 
+#if IS_ENABLED(CONFIG_NFS_V4_2)
 int
 pnfs_report_layoutstat(struct inode *inode)
 {
@@ -2306,3 +2307,4 @@ out_put:
 	goto out;
 }
 EXPORT_SYMBOL_GPL(pnfs_report_layoutstat);
+#endif