IB/rxe: remove unused function variable

In the functions rxe_mem_init_dma, rxe_mem_init_user, rxe_mem_init_fast
and copy_data, the function variable rxe is not used. So this function
variable rxe is removed.

CC: Srinivas Eeda <srinivas.eeda@oracle.com>
CC: Junxiao Bi <junxiao.bi@oracle.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>