IB/rxe: Change RDMA_RXE kconfig to use select

NET_UDP_TUNNEL is not user selectable, so it should be used as a select
in kconfig.

CRYPTO_CRC32 is a required library for RDMA_RXE so it should active
automatically, as most other CRYPTO_ users do.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>