lib/rhashtable: fix race between rhashtable_lookup_compare and hashtable resize

lib/rhashtable: fix race between rhashtable_lookup_compare and hashtable resize

Hash value passed as argument into rhashtable_lookup_compare could be
computed using different hash table than rhashtable_lookup_compare sees.

This patch passes key into rhashtable_lookup_compare() instead of hash and
compures hash value right in place using the same table as for lookup.

Also it adds comment for rhashtable_hashfn and rhashtable_obj_hashfn:
user must prevent concurrent insert/remove otherwise returned hash value
could be invalid.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Fixes: e341694e3eb5 ("netlink: Convert netlink_lookup() to use RCU protected hash table")
Link: http://lkml.kernel.org/r/20150514042151.GA5482@gondor.apana.org.au
Cc: Stable <stable@vger.kernel.org> (v3.17 .. v3.19)
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>