Fix LinearSolverIstl cleanup also for fast AMG branch

To avoid deprecation warnings the number of smoothing steps was passed
through the Criterion instead of directly to the constructor in commit
a7f32b934b.

However, due to an insufficient test matrix this was not tested using
the fast AMG variant of DUNE so it breaks the builds if
`-DHAS_DUNE_FAST_AMG` is defined.

This change should apply the same type of change to this branch as for
the others. The number of smoothing steps is put into a constant to
avoid the magic number 1 to appear in too many places (although I am
not sure the number for pre- and post-smoothing always should be the
same).
Merge pull request #379 from rolk/379_famg

Fix LinearSolverIstl cleanup also for fast AMG branch