MAINTAINERS: Clean RXE section and add Zhu as RXE maintainer

Zhu Yanjun contributed many patches to RXE and expressed genuine interest
in improve RXE even more. Let's add him as a maintainer.

Link: https://lore.kernel.org/r/20200312083658.29603-1-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Acked-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>