af_unix: Don't call wait_for_unix_gc() on every sendmsg().

af_unix: Don't call wait_for_unix_gc() on every sendmsg().

We have been calling wait_for_unix_gc() on every sendmsg() in case
there are too many inflight AF_UNIX sockets.

This is also because the old GC implementation had poor knowledge
of the inflight sockets and had to suspect every sendmsg().

This was improved by commit d9f21b361333 ("af_unix: Try to run GC
async."), but we do not even need to call wait_for_unix_gc() if the
process is not sending AF_UNIX sockets.

The wait_for_unix_gc() call only helps when a malicious process
continues to create cyclic references, and we can detect that
in a better place and slow it down.

Let's move wait_for_unix_gc() to unix_prepare_fpl() that is called
only when AF_UNIX socket fd is passed via SCM_RIGHTS.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20251115020935.2643121-5-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>