IB/rxe: Fix up rxe_qp_cleanup()

Replace sk_dst_get()/dst_release() in rxe_qp_cleanup() with sk_dst_reset().
sk_dst_get() takes a new reference on dst, so the dst_release() doesn't
actually release the original reference, which was the design intent.

Fixes: 4ed6ad1e ("IB/rxe: Cache dst in QP instead of getting it...")
Signed-off-by: Andrew Boyer <andrew.boyer@dell.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>