Add manpage for mlx4ib(4)

This manpage isn't differentiated from mlx4en except where necessary,
replacing eth/ETH with ib/IB.

Eventually the manpages will be split and the common bits be placed
in a manpage named "mlx4.4".

MFC after: 3 weeks
Reviewed by: hselasky
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D9241