get rid of CL_SHARE_TO_SLAVE

the only difference between it and CL_SLAVE is in this predicate
in clone_mnt():
	if ((flag & CL_SLAVE) ||
	    ((flag & CL_SHARED_TO_SLAVE) && IS_MNT_SHARED(old))) {
However, in case of CL_SHARED_TO_SLAVE we have not allocated any
mount group ids since the time we'd grabbed namespace_sem, so
IS_MNT_SHARED() is equivalent to non-zero ->mnt_group_id.  And
in case of CL_SLAVE old has come either from the original tree,
which had ->mnt_group_id allocated for all nodes or from result
of sequence of CL_MAKE_SHARED or CL_MAKE_SHARED|CL_SLAVE copies,
ultimately going back to the original tree.  In both cases we are
guaranteed that old->mnt_group_id will be non-zero.

In other words, the predicate is always equal to
	(flags & (CL_SLAVE | CL_SHARED_TO_SLAVE)) && old->mnt_group_id
and with that replacement CL_SLAVE and CL_SHARED_TO_SLAVE have exact
same behaviour.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Author: Al Viro

fs/namespace.c

@@ -1309,7 +1309,7 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
 	mnt->mnt.mnt_flags = READ_ONCE(old->mnt.mnt_flags) &
 			     ~MNT_INTERNAL_FLAGS;
 
-	if (flag & (CL_SLAVE | CL_PRIVATE | CL_SHARED_TO_SLAVE))
+	if (flag & (CL_SLAVE | CL_PRIVATE))
 		mnt->mnt_group_id = 0; /* not a peer of original */
 	else
 		mnt->mnt_group_id = old->mnt_group_id;
@@ -1340,8 +1340,7 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
 	if (peers(mnt, old))
 		list_add(&mnt->mnt_share, &old->mnt_share);
 
-	if ((flag & CL_SLAVE) ||
-	    ((flag & CL_SHARED_TO_SLAVE) && IS_MNT_SHARED(old))) {
+	if ((flag & CL_SLAVE) && old->mnt_group_id) {
 		hlist_add_head(&mnt->mnt_slave, &old->mnt_slave_list);
 		mnt->mnt_master = old;
 	} else if (IS_MNT_SLAVE(old)) {
@@ -4228,7 +4227,7 @@ struct mnt_namespace *copy_mnt_ns(unsigned long flags, struct mnt_namespace *ns,
 	/* First pass: copy the tree topology */
 	copy_flags = CL_COPY_UNBINDABLE | CL_EXPIRE;
 	if (user_ns != ns->user_ns)
-		copy_flags |= CL_SHARED_TO_SLAVE;
+		copy_flags |= CL_SLAVE;
 	new = copy_tree(old, old->mnt.mnt_root, copy_flags);
 	if (IS_ERR(new)) {
 		namespace_unlock();

fs/pnode.h

@@ -25,7 +25,6 @@
 #define CL_COPY_UNBINDABLE	0x04
 #define CL_MAKE_SHARED 		0x08
 #define CL_PRIVATE 		0x10
-#define CL_SHARED_TO_SLAVE	0x20
 #define CL_COPY_MNT_NS_FILE	0x40
 
 /*