nfsd: fix compose_entry_fh() failure exits

Restore the original logics ("fail on mountpoints, negatives and in
case of fh_compose() failures").  Since commit 8177e (nfsd: clean up
readdirplus encoding) that got broken -
	rv = fh_compose(fhp, exp, dchild, &cd->fh);
	if (rv)
	       goto out;
	if (!dchild->d_inode)
		goto out;
	rv = 0;
out:
is equivalent to
	rv = fh_compose(fhp, exp, dchild, &cd->fh);
out:
and the second check has no effect whatsoever...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>