RDMA/core/multicast: Provide description for 'ib_init_ah_from_mcmember()'s 'rec'...

RDMA/core/multicast: Provide description for 'ib_init_ah_from_mcmember()'s 'rec' param

Fixes the following W=1 kernel build warning(s):

 drivers/infiniband/core/multicast.c:739: warning: Function parameter or member 'rec' not described in 'ib_init_ah_from_mcmember'

Link: https://lore.kernel.org/r/20210118223929.512175-12-lee.jones@linaro.org
Cc: Doug Ledford <dledford@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>