In the previous GC implementation, the shape of the inflight socket
graph was not expected to change while GC was in progress.

MSG_PEEK was tricky because it could install inflight fd silently
and transform the graph.

Let's say we peeked a fd, which was a listening socket, and accept()ed
some embryo sockets from it.  The garbage collection algorithm would
have been confused because the set of sockets visited in scan_inflight()
would change within the same GC invocation.

That's why we placed spin_lock(&unix_gc_lock) and spin_unlock() in
unix_peek_fds() with a fat comment.

In the new GC implementation, we no longer garbage-collect the socket
if it exists in another queue, that is, if it has a bridge to another
SCC.  Also, accept() will require the lock if it has edges.

Thus, we need not do the complicated lock dance.
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240401173125.92184-3-kuniyu@amazon.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>