RDMA/core: Use sizeof_field() helper

Make use of the sizeof_field() helper instead of an open-coded version.

Link: https://lore.kernel.org/r/20200527144152.GA22605@embeddedor


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>