staging: lustre: convert lov_pool to use rhashtable
The pools hashtable can be implemented using
the rhashtable implementation in lib.
This has the benefit that lookups are lock-free.
We need to use kfree_rcu() to free a pool so
that a lookup racing with a deletion will not access
freed memory.
rhashtable has no combined lookup-and-delete interface,
but as the lookup is lockless and the chains are short,
this brings little cost. Even if a lookup finds a pool,
we must be prepared for the delete to fail to find it,
as we might race with another thread doing a delete.
We use atomic_inc_not_zero() after finding a pool in the
hash table and if that fails, we must have raced with a
deletion, so we treat the lookup as a failure.
Use hashlen_string() rather than a hand-crafted hash
function.
Note that the pool_name, and the search key, are
guaranteed to be nul terminated.