RDMA/rxe: Replace missing rxe_pool_get_index_locked

One of the pool APIs for when caller is holding lock was not defined but
is declared in rxe_pool.h. This patch adds the definition.

Link: https://lore.kernel.org/r/20210125211641.2694-7-rpearson@hpe.com
Signed-off-by: Bob Pearson <rpearson@hpe.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>