datapath: Improve kernel hash table

datapath: Improve kernel hash table

 Currently OVS uses its own hashing implmentation for hash tables
which has some problems, e.g. error case on deletion code.
Following patch replaces that with hlist based hash table which is
consistent with other kernel hash tables. As Jesse suggested, flex-array
is used for allocating hash buckets, So that we can have large
hash-table without large contiguous kernel memory.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>