vfs: Remove d_drop calls from d_revalidate implementations

Now that d_invalidate always succeeds it is not longer necessary or
desirable to hard code d_drop calls into filesystem specific
d_revalidate implementations.

Remove the unnecessary d_drop calls and rely on d_invalidate
to drop the dentries.  Using d_invalidate ensures that paths
to mount points will not be dropped.

Reviewed-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/ceph/dir.c

@@ -1069,7 +1069,6 @@ static int ceph_d_revalidate(struct dentry *dentry, unsigned int flags)
 		ceph_dentry_lru_touch(dentry);
 	} else {
 		ceph_dir_clear_complete(dir);
-		d_drop(dentry);
 	}
 	iput(dir);
 	return valid;