IB/rxe: double free on error

"goto err;" has it's own kfree_skb() call so it's a double free.  We
only need to free on the "goto exit;" path.

Fixes: 8700e3e7

 ("Soft RoCE driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>