See #645527.

Our former userspace allows 108 bytes in sun_path,
but kernel restrict it to 104 bytes.

Userland compatibility was added in glibc 2.13-22 (glibc-bsd r3739). This
patch can be removed when we no longer care about supporting older glibc.

--- a/sys/kern/uipc_usrreq.c
+++ b/sys/kern/uipc_usrreq.c
@@ -474,7 +474,12 @@
 	KASSERT(unp != NULL, ("uipc_bind: unp == NULL"));
 
 	if (soun->sun_len > sizeof(struct sockaddr_un))
+	{
+	    if (soun->sun_len <= (4 + sizeof(struct sockaddr_un)))
+	        soun->sun_len = sizeof(struct sockaddr_un);  	/* clip it */
+	    else
 		return (EINVAL);
+	};
 	namelen = soun->sun_len - offsetof(struct sockaddr_un, sun_path);
 	if (namelen <= 0)
 		return (EINVAL);
@@ -1290,7 +1295,12 @@
 	KASSERT(unp != NULL, ("unp_connect: unp == NULL"));
 
 	if (nam->sa_len > sizeof(struct sockaddr_un))
+	{
+	    if (nam->sa_len <= (4 + sizeof(struct sockaddr_un)))
+	        nam->sa_len = sizeof(struct sockaddr_un);	/* clip it */
+	    else
 		return (EINVAL);
+	};
 	len = nam->sa_len - offsetof(struct sockaddr_un, sun_path);
 	if (len <= 0)
 		return (EINVAL);