[IPoIB] don't compile debug code if debugging isn't enabled

Don't build ipoib_mcast_iter_ functions if CONFIG_INFINIBAND_IPOIB_DEBUG
is not enabled -- their only callers will not be built either.

Also move the prototype for ipoib_open() to ipoib.h to fix a sparse warning.

Signed-off-by: Roland Dreier <rolandd@cisco.com>