get rid of lustre_dump_dentry()

it's not only badly racy, it's actually dead code - the call in
ll_invalidate_aliases() is unreachable.  For an alias of our inode
to be root dentry, the inode would have to be its ->d_inode, aka
the root inode, and we never call ll_invalidate_aliases() for that.

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