vfs: new internal helper: mnt_has_parent(mnt)

vfsmounts have ->mnt_parent pointing either to a different vfsmount
or to itself; it's never NULL and termination condition in loops
traversing the tree towards root is mnt == mnt->mnt_parent.  At least
one place (see the next patch) is confused about what's going on;
let's add an explicit helper checking it right way and use it in
all places where we need it.  Not that there had been too many,
but...

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

fs/pnode.h

@@ -9,7 +9,7 @@
 #define _LINUX_PNODE_H
 
 #include <linux/list.h>
-#include <linux/mount.h>
+#include "mount.h"
 
 #define IS_MNT_SHARED(mnt) (mnt->mnt_flags & MNT_SHARED)
 #define IS_MNT_SLAVE(mnt) (mnt->mnt_master)