[!245] [gmres][fix] Do not restart on last iteration

Merge branch 'fix/gmres-dont-restart-on-last-iteration' into 'master'

ref:core/dune-istl Kudos to @mathis.springwald for pointing this out.

See merge request [!245]

  [!245]: gitlab.dune-project.org/core/dune-istl/merge_requests/245