NFSD: Fix trace_nfsd_fh_verify_err() crasher

Now that the nfsd_fh_verify_err() tracepoint is always called on
error, it needs to handle cases where the filehandle is not yet
fully formed.

Fixes: 93c128e7 ("nfsd: ensure we always call fh_verify_error tracepoint")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>

fs/nfsd/trace.h

@@ -254,7 +254,10 @@ TRACE_EVENT_CONDITION(nfsd_fh_verify_err,
 				  rqstp->rq_xprt->xpt_remotelen);
 		__entry->xid = be32_to_cpu(rqstp->rq_xid);
 		__entry->fh_hash = knfsd_fh_hash(&fhp->fh_handle);
-		__entry->inode = d_inode(fhp->fh_dentry);
+		if (fhp->fh_dentry)
+			__entry->inode = d_inode(fhp->fh_dentry);
+		else
+			__entry->inode = NULL;
 		__entry->type = type;
 		__entry->access = access;
 		__entry->error = be32_to_cpu(error);