neighbour: Allocate skb in neigh_get().

We will remove RTNL for neigh_get() and run it under RCU instead.

neigh_get_reply() and pneigh_get_reply() allocate skb with GFP_KERNEL.

Let's move the allocation before __dev_get_by_index() in neigh_get().

Now, neigh_get_reply() and pneigh_get_reply() are inlined and
rtnl_unicast() is factorised.

We will convert pneigh_lookup() to __pneigh_lookup() later.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250716221221.442239-4-kuniyu@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>