RDMA/rxe: Lookup kernel AH from ah index in UD WQEs

Add code to rxe_get_av in rxe_av.c to use the AH index in UD send WQEs to
lookup the kernel AH. For old user providers continue to use the AV passed
in WQEs. Move setting pkt->rxe to before the call to rxe_get_av() to get
access to the AH pool.

Link: https://lore.kernel.org/r/20211007204051.10086-6-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>