  Gamma-ray burst afterglows arise from relativistic collisionless shocks in
which the postshock tangled magnetic field $\vec{B}$ is produced by the
two-stream and/or Weibel instabilities on plasma skin-depth scales
$(c/\omega_p)$. The field is expected to be oriented predominantly within the
shock plane ($B_{\perp}$; transverse to the shock normal, $\hat{n}_{\rm{sh}}$),
and is often approximated to be completely within it
($B_\parallel\equiv\hat{n}_{\rm{sh}}\,\cdot\,\vec{B}=0$). Current 2D/3D
particle-in-cell simulations are limited to short timescales and box sizes
$\lesssim10^4(c/\omega_p)\ll R/\Gamma_{\rm{sh}}$ much smaller than the shocked
region's comoving width, and cannot probe the asymptotic downstream $\vec{B}$
structure. We constrain the latter using the linear polarization upper limit,
$|\Pi|<12\%$, on the radio afterglow of GW170817/GRB170817A. Afterglow
polarization depends on the jet's angular structure, our viewing angle, and the
$\vec{B}$ structure. In GW170817/GRB170817A the latter can be tightly
constrained since the former two are constrained from observations. We model
$\vec{B}$ as an isotropic field in 3D that is stretched along
$\hat{n}_{\rm{sh}}$ by a factor $\xi\equiv{}B_{\parallel}/B_{\perp}$, whose
initial value $\xi_f\equiv{}B_{\parallel,f}/B_{\perp,f}$ describes the field
that survives downstream on plasma scales $\ll{}R/\Gamma_{\rm{sh}}$. We
calculate $\Pi(\xi_f)$ by integrating over the shocked volume for
core-dominated structured jets, with a local Blandford-McKee self-similar
radial profile. We find that independent of the exact jet structure, $\vec{B}$
has a finite, but initially sub-dominant, parallel component:
$0.57\lesssim\xi_f\lesssim0.89$, making it less anisotropic. While this
motivates numerical studies of the asymptotic $\vec{B}$ structure in
relativistic collisionless shocks, it may be consistent with turbulence
amplified magnetic field.
