af_unix: Allocate struct unix_edge for each inflight AF_UNIX fd.

As with the previous patch, we preallocate to skb's scm_fp_list an
array of struct unix_edge in the number of inflight AF_UNIX fds.

There we just preallocate memory and do not use immediately because
sendmsg() could fail after this point.  The actual use will be in
the next patch.

When we queue skb with inflight edges, we will set the inflight
socket's unix_sock as unix_edge->predecessor and the receiver's
unix_sock as successor, and then we will link the edge to the
inflight socket's unix_vertex.edges.

Note that we set NULL to cloned scm_fp_list.edges in scm_fp_dup()
so that MSG_PEEK does not change the shape of the directed graph.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240325202425.60930-3-kuniyu@amazon.com


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