IB/rxe: replace refcount_inc with skb_get

Follow the advice from Bart, the function refcount_inc is replaced
with skb_get in commit 99dae690 ("IB/rxe: optimize mcast recv process")
and commit 86af6176 ("IB/rxe: remove unnecessary skb_clone").

CC: Srinivas Eeda <srinivas.eeda@oracle.com>
CC: Junxiao Bi <junxiao.bi@oracle.com>
Suggested-by: Bart Van Assche <Bart.VanAssche@wdc.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>