staging: lustre: convert osc_quota hash to rhashtable

As this is indexed by an integer, an extensible array
or extensible bitmap would be better.
If/when xarray lands, we should change to use that.

For now, just a simple conversion to rhashtable.

When removing an entry, we need to hold rcu_read_lock()
across the lookup and remove in case we race with another thread
performing a removal.  This means we need to use call_rcu()
to free the quota info so we need an rcu_head in there, which
unfortunately doubles the size of the structure.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>