Allocate a zeroed LDT.

Failing to do this might result in the LDT appearing to run out of free
descriptors because of random junk in the descriptor's 'sd_type' field.

http://lists.freebsd.org/pipermail/freebsd-amd64/2014-May/016088.html

Reviewed by:	kib
MFC after:	2 weeks

sys/amd64/amd64/sys_machdep.c

@@ -462,7 +462,7 @@ user_ldt_alloc(struct proc *p, int force)
 	new_ldt = malloc(sizeof(struct proc_ldt), M_SUBPROC, M_WAITOK);
 	new_ldt->ldt_base = (caddr_t)kmem_malloc(kernel_arena,
 	     max_ldt_segment * sizeof(struct user_segment_descriptor),
-	     M_WAITOK);
+	     M_WAITOK | M_ZERO);
 	if (new_ldt->ldt_base == NULL) {
 		FREE(new_ldt, M_SUBPROC);
 		mtx_lock(&dt_lock);