VFS: Clean up shared mount flag propagation

The handling of mount flags in set_mnt_shared() got a little tangled
up during previous cleanups, with the following problems:

* MNT_PNODE_MASK is defined as a literal constant when it should be a
bitwise xor of other MNT_* flags
* set_mnt_shared() clears and then sets MNT_SHARED (part of MNT_PNODE_MASK)
* MNT_PNODE_MASK could use a comment in mount.h
* MNT_PNODE_MASK is a terrible name, change to MNT_SHARED_MASK

This patch fixes these problems.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/namespace.c b/fs/namespace.c
index 25c1dcf..d25d460 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1538,7 +1538,7 @@
 		err = do_remount_sb(sb, flags, data, 0);
 	if (!err) {
 		spin_lock(&vfsmount_lock);
-		mnt_flags |= path->mnt->mnt_flags & MNT_PNODE_MASK;
+		mnt_flags |= path->mnt->mnt_flags & MNT_PROPAGATION_MASK;
 		path->mnt->mnt_flags = mnt_flags;
 		spin_unlock(&vfsmount_lock);
 	}