netfilter: ipset: hash:ip set type support

The module implements the hash:ip type support in four flavours:
for IPv4 or IPv6, both without and with timeout support.

All the hash types are based on the "array hash" or ahash structure
and functions as a good compromise between minimal memory footprint
and speed. The hashing uses arrays to resolve clashes. The hash table
is resized (doubled) when searching becomes too long. Resizing can be
triggered by userspace add commands only and those are serialized by
the nfnl mutex. During resizing the set is read-locked, so the only
possible concurrent operations are the kernel side readers. Those are
protected by RCU locking.

Because of the four flavours and the other hash types, the functions
are implemented in general forms in the ip_set_ahash.h header file
and the real functions are generated before compiling by macro expansion.
Thus the dereferencing of low-level functions and void pointer arguments
could be avoided: the low-level functions are inlined, the function
arguments are pointers of type-specific structures.

Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>