neighbour: Update pneigh_entry in pneigh_create().

neigh_add() updates pneigh_entry() found or created by pneigh_create().

This update is serialised by RTNL, but we will remove it.

Let's move the update part to pneigh_create() and make it return errno
instead of a pointer of pneigh_entry.

Now, the pneigh code is RTNL free.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250716221221.442239-16-kuniyu@google.com


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