IB/rxe: avoid calling WARN_ON_ONCE twice

In the exit branch, WARN_ON_ONCE is called to show stack. So it is
not necessary to call WARN_ON_ONCE before going to exit.

Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>