support madvise(MADV_REMOVE) partially

This MADV_REMOVE works with a mapping which is
- created with shmat() and
- not sharing memobj with other mappings.

kernel/include/memobj.h

@@ -31,6 +31,7 @@ enum {
 	/* for memobj.flags */
 	MF_HAS_PAGER	= 0x0001,
 	MF_SHMDT_OK	= 0x0002,
+	MF_IS_REMOVABLE	= 0x0004,
 };
 
 struct memobj {
@@ -120,6 +121,11 @@ static inline int memobj_has_pager(struct memobj *obj)
 	return !!(obj->flags & MF_HAS_PAGER);
 }
 
+static inline int memobj_is_removable(struct memobj *obj)
+{
+	return !!(obj->flags & MF_IS_REMOVABLE);
+}
+
 int fileobj_create(int fd, struct memobj **objp, int *maxprotp);
 struct shmid_ds;
 int shmobj_create(struct shmid_ds *ds, struct memobj **objp);