IB/IPoIB: factor out common multicast list removal code

Code cleanup to remove multicast specific code from ipoib_main.c

The removal of a list of multicast groups occurs in three places.
Create a new function ipoib_mcast_remove_list(). Use this new
function in ipoib_main.c too.
That in turn allows the dropping of two functions that were
exported from ipoib_multicast.c for expiration of mc groups.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>