RDMA/rxe: Fix over copying in get_srq_wqe

Currently get_srq_wqe() in rxe_resp.c copies the maximum possible number
of bytes from the wqe into the QPs copy of the SRQ wqe. This is usually
extra work and risks reading past the end of the SRQ circular buffer if
the SRQ is configured with less than the maximum possible number of SGEs.

Check the number of SGEs is not too large.
Compute the actual number of bytes in the WR and copy only those.

Fixes: 8700e3e7

 ("Soft RoCE driver")
Link: https://lore.kernel.org/r/20210618045742.204195-5-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>