RDMA/rxe: Move ICRC generation to a subroutine

Isolate ICRC generation into a single subroutine named rxe_generate_icrc()
in rxe_icrc.c. Remove scattered crc generation code from elsewhere.

Link: https://lore.kernel.org/r/20210707040040.15434-5-rpearsonhpe@gmail.com


Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>