IB/rxe: Make function rxe_pool_cleanup return void

Since the function always returns 0 make it void.

Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>