epair: unbind prior to returning to userspace

If 'options RSS' is set we bind the epair tasks to different CPUs. We
must take care to not keep the current thread bound to the last CPU when
we return to userspace.

MFC after: 1 week
Sponsored by: Orange Business Services

(cherry picked from commit cbbce42345c51536d7dc63643f47a3cbf87d647e)