fdescfs: Set *eofflag in fdesc_readdir()

[kib]

sys/fs/fdescfs/fdesc_vnops.c

564–569

Might be write this without duplication?

while (uio->uio_resid >= UIO_MX) {
     if (i >= fdp->fd_nfiles + 2) {
           if (ap->a_eofflag != NULL)
             *ap->a_eofflag = 1;
          break;
    }

[markj]

Deduplicate