IB/rxe: move the variable into the function that uses it

The variable rxe is only used in the function rxe_xmit_packet, and the
caller functions do not use it. So move this variable into the function
rxe_xmit_packet.

Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>