dcache: touch up predicts in __d_lookup_rcu()

dcache: touch up predicts in __d_lookup_rcu()

Rationale is that if the parent dentry is the same and the length is the
same, then you have to be unlucky for the name to not match.

At the same time the dentry was literally just found on the hash, so you
have to be even more unlucky to determine it is unhashed.

While here add commentary while d_unhashed() is necessary. It was
already removed once and brought back in:
2e321806b681b192 ("Revert "vfs: remove unnecessary d_unhashed() check from __d_lookup_rcu"")

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251127131526.4137768-1-mjguzik@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>