Add start_renaming_two_dentries()

Add start_renaming_two_dentries()

A few callers want to lock for a rename and already have both dentries.
Also debugfs does want to perform a lookup but doesn't want permission
checking, so start_renaming_dentry() cannot be used.

This patch introduces start_renaming_two_dentries() which is given both
dentries.  debugfs performs one lookup itself.  As it will only continue
with a negative dentry and as those cannot be renamed or unlinked, it is
safe to do the lookup before getting the rename locks.

overlayfs uses start_renaming_two_dentries() in three places and  selinux
uses it twice in sel_make_policy_nodes().

In sel_make_policy_nodes() we now lock for rename twice instead of just
once so the combined operation is no longer atomic w.r.t the parent
directory locks.  As selinux_state.policy_mutex is held across the whole
operation this does not open up any interesting races.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20251113002050.676694-13-neilb@ownmail.net
Signed-off-by: Christian Brauner <brauner@kernel.org>