solve_cg: use linop.make instead of MF constructor (#3627)

## Summary

This PR replaces the explicit use of `MF` constructors in
`MLCGSolverT<MF>::solve_cg` with calls to the make method of the linear
operator associated with the `MLCGSolverT` object.

## Additional background

This is a similar to the PR on `solve_bicgstab`.

Src/LinearSolvers/MLMG/AMReX_MLCGSolver.H

@@ -261,17 +261,13 @@ MLCGSolverT<MF>::solve_cg (MF& sol, const MF& rhs, RT eps_rel, RT eps_abs)
 
     const int ncomp = sol.nComp();
 
-    const BoxArray& ba = sol.boxArray();
-    const DistributionMapping& dm = sol.DistributionMap();
-    const auto& factory = sol.Factory();
-
-    MF p(ba, dm, ncomp, sol.nGrowVect(), MFInfo(), factory);
+    MF p = Lp.make(amrlev, mglev, sol.nGrowVect());
     p.setVal(RT(0.0));
 
-    MF sorig(ba, dm, ncomp, nghost, MFInfo(), factory);
-    MF r    (ba, dm, ncomp, nghost, MFInfo(), factory);
-    MF z    (ba, dm, ncomp, nghost, MFInfo(), factory);
-    MF q    (ba, dm, ncomp, nghost, MFInfo(), factory);
+    MF sorig = Lp.make(amrlev, mglev, nghost);
+    MF r     = Lp.make(amrlev, mglev, nghost);
+    MF z     = Lp.make(amrlev, mglev, nghost);
+    MF q     = Lp.make(amrlev, mglev, nghost);
 
     sorig.LocalCopy(sol,0,0,ncomp,nghost);