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/pnode.h b/fs/pnode.h
index 6c7ef32..1ea4ae1e 100644
--- a/fs/pnode.h
+++ b/fs/pnode.h
@@ -25,7 +25,7 @@
 
 static inline void set_mnt_shared(struct vfsmount *mnt)
 {
-	mnt->mnt_flags &= ~MNT_PNODE_MASK;
+	mnt->mnt_flags &= ~MNT_SHARED_MASK;
 	mnt->mnt_flags |= MNT_SHARED;
 }