vfs: start hiding vfsmount guts series

Almost all fields of struct vfsmount are used only by core VFS (and
a fairly small part of it, at that).  The plan: embed struct vfsmount
into struct mount, making the latter visible only to core parts of VFS.
Then move fields from vfsmount to mount, eventually leaving only
mnt_root/mnt_sb/mnt_flags in struct vfsmount.  Filesystem code still
gets pointers to struct vfsmount and remains unchanged; all such
pointers go to struct vfsmount embedded into the instances of struct
mount allocated by fs/namespace.c.  When fs/namespace.c et.al. get
a pointer to vfsmount, they turn it into pointer to mount (using
container_of) and work with that.

This is the first part of series; struct mount is introduced,
allocation switched to using it.

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

fs/mount.h

@@ -1,5 +1,14 @@
 #include <linux/mount.h>
 
+struct mount {
+	struct vfsmount mnt;
+};
+
+static inline struct mount *real_mount(struct vfsmount *mnt)
+{
+	return container_of(mnt, struct mount, mnt);
+}
+
 static inline int mnt_has_parent(struct vfsmount *mnt)
 {
 	return mnt != mnt->mnt_parent;