Make the gid argument to ibv_attach_mcast and ibv_detach_mcast const

Make the gid argument to ibv_attach_mcast and ibv_detach_mcast const

ibv_attach_mcast() and ibv_detach_mcast() don't change the gid
argument, so the arguments should be const to allow applications to
pass in constant gids.  This constness flows through to the driver
call struct and into the drivers and back into
ibv_cmd_attach_mcast()/ibv_cmd_detach_mcast().

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>