RDMA/netlink: Avoid double pass for RDMA netlink messages

The standard netlink_rcv_skb function skips messages without
NLM_F_REQUEST flag in it, while SA netlink client issues them.

In commit bc10ed7d ("IB/core: Add rdma netlink helper functions")
the local function was introduced to allow such messages.

This led to double pass for every incoming message.

In this patch, we unify that local implementation and netlink_rcv_skb
functions, so there will be no need for double pass anymore.

As a outcome, this combined function gained more strict check
for NLM_F_REQUEST flag and it is now allowed for SA pathquery
client only.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>