Fix handling of errors from sblock() in soreceive_stream().

Previously we would attempt to unlock the socket buffer despite having
failed to lock it.  Simply return an error instead: no resources need
to be released at this point, and doing so is consistent with
soreceive_generic().

PR:		238789
Submitted by:	Greg Becker <greg@codeconcepts.com>
MFC after:	1 week

sys/kern/uipc_socket.c

@@ -2200,7 +2200,7 @@ soreceive_stream(struct socket *so, struct sockaddr **psa, struct uio *uio,
 	/* Prevent other readers from entering the socket. */
 	error = sblock(sb, SBLOCKWAIT(flags));
 	if (error)
-		goto out;
+		return (error);
 	SOCKBUF_LOCK(sb);
 
 	/* Easy one, no space to copyout anything. */