afs: Use the contained hashtable to search a directory

afs: Use the contained hashtable to search a directory

Each directory image contains a hashtable with 128 buckets to speed up
searching.  Currently, kafs does not use this, but rather iterates over all
the occupied slots in the image as it can share this with readdir.

Switch kafs to use the hashtable for lookups to reduce the latency.  Care
must be taken that the hash chains are acyclic.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20241216204124.3752367-30-dhowells@redhat.com
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Christian Brauner <brauner@kernel.org>