get rid of ->mnt_longterm

it's enough to set ->mnt_ns of internal vfsmounts to something
distinct from all struct mnt_namespace out there; then we can
just use the check for ->mnt_ns != NULL in the fast path of
mntput_no_expire()

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

fs/mount.h

@@ -22,7 +22,6 @@ struct mount {
 	struct vfsmount mnt;
 #ifdef CONFIG_SMP
 	struct mnt_pcp __percpu *mnt_pcp;
-	atomic_t mnt_longterm;		/* how many of the refs are longterm */
 #else
 	int mnt_count;
 	int mnt_writers;
@@ -49,6 +48,8 @@ struct mount {
 	int mnt_ghosts;
 };
 
+#define MNT_NS_INTERNAL ERR_PTR(-EINVAL) /* distinct from any mnt_namespace */
+
 static inline struct mount *real_mount(struct vfsmount *mnt)
 {
 	return container_of(mnt, struct mount, mnt);
@@ -59,6 +60,12 @@ static inline int mnt_has_parent(struct mount *mnt)
 	return mnt != mnt->mnt_parent;
 }
 
+static inline int is_mounted(struct vfsmount *mnt)
+{
+	/* neither detached nor internal? */
+	return !IS_ERR_OR_NULL(real_mount(mnt));
+}
+
 extern struct mount *__lookup_mnt(struct vfsmount *, struct dentry *, int);
 
 static inline void get_mnt_ns(struct mnt_namespace *ns)