Do not collapse objects with OBJ_NOSPLIT backing swap object.

NOSPLIT swap objects are not anonymous, they are used by tmpfs regular
files and POSIX shared memory.  For such objects, collapse is not
permitted.

Reported by:	mjg
Reviewed by:	markj, trasz
Tested by:	mjg, pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D19923

sys/vm/vm_object.c

@@ -1748,7 +1748,8 @@ vm_object_collapse(vm_object_t object)
 		VM_OBJECT_WLOCK(backing_object);
 		if (backing_object->handle != NULL ||
 		    (backing_object->type != OBJT_DEFAULT &&
-		     backing_object->type != OBJT_SWAP) ||
+		    (backing_object->type != OBJT_SWAP ||
+		    (backing_object->flags & OBJ_NOSPLIT) != 0)) ||
 		    (backing_object->flags & OBJ_DEAD) ||
 		    object->handle != NULL ||
 		    (object->type != OBJT_DEFAULT &&