af_unix: fix struct pid memory leak

Dmitry reported a struct pid leak detected by a syzkaller program.

Bug happens in unix_stream_recvmsg() when we break the loop when a
signal is pending, without properly releasing scm.

Fixes: b3ca9b0 ("net: fix multithreaded signal handling in unix recv routines")
Reported-by: Dmitry Vyukov <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Cc: Rainer Weikusat <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

net/unix/af_unix.c

@@ -2339,6 +2339,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state)
 
 			if (signal_pending(current)) {
 				err = sock_intr_errno(timeo);
+				scm_destroy(&scm);
 				goto out;
 			}