NFSv4: Fix a tracepoint Oops in initiate_file_draining()

Now that the value of 'ino' can be NULL or an ERR_PTR(), we need to
change the test in the tracepoint.

Fixes: ce5624f7e6 ("NFSv4: Return NFS4ERR_DELAY when a layout fails...")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: stable@vger.kernel.org # v4.17+
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/nfs4trace.h

@@ -1137,7 +1137,7 @@ DECLARE_EVENT_CLASS(nfs4_inode_callback_event,
 		TP_fast_assign(
 			__entry->error = error;
 			__entry->fhandle = nfs_fhandle_hash(fhandle);
-			if (inode != NULL) {
+			if (!IS_ERR_OR_NULL(inode)) {
 				__entry->fileid = NFS_FILEID(inode);
 				__entry->dev = inode->i_sb->s_dev;
 			} else {