Fix kernel memory disclosure in ibcs2_getdents

ibcs2_getdents() copies a dirent structure to userland.  The ibcs2
dirent structure contains a 2 byte pad element.  This element is never
initialized, but copied to userland none-the-less.

Note that ibcs2 has not built on HEAD since r302095.

Submitted by:	Domagoj Stolfa <ds815@cam.ac.uk>
Reported by:	Ilja Van Sprundel <ivansprundel@ioactive.com>
MFC after:	3 days
Security:	Kernel memory disclosure (803)

sys/i386/ibcs2/ibcs2_misc.c

@@ -330,6 +330,7 @@ ibcs2_getdents(struct thread *td, struct ibcs2_getdents_args *uap)
 #define	BSD_DIRENT(cp)		((struct dirent *)(cp))
 #define	IBCS2_RECLEN(reclen)	(reclen + sizeof(u_short))
 
+	memset(&idb, 0, sizeof(idb));
 	error = getvnode(td, uap->fd, cap_rights_init(&rights, CAP_READ), &fp);
 	if (error != 0)
 		return (error);