The summary of what the patent covers is not very good here. The Nemes patents (all filed in the last century) all involve hash-indexed records becoming obsolete in some way but not being deleted at once for some reason. Instead, the records are removed opportunistically when the linked list of records having the same hash is traversed for some other reason (including the explicit deletion of one of them).
As far as I can tell, that is the novelty that is not dealt with in any of the extensive, existing work on hash-based searching of records.
I have no idea what the use is in the Linux kernel that would make it difficult to work around this, but I can think of two factors:
1. Allowing records to expire or to be marked as expired for later garbage collection could be a way to avoid locking overhead and issues.
2. It might be difficult to know and/or to keep track of the linked list that the record (the significant data, not necessarily in the linked-list entry itself) is actually on.
In any case, I suspect it will be important to work around the patent and to do so quickly.