Staging: batman-adv: Move hash callback related function to header

To enable inlining of the function pointers hashdata_choose_cb,
hashdata_choose_cb and hashdata_free_cb, also the hash functions which
uses them must be inlined by the called function.

This should increase the performance, but also increases the size of the
generated machine code slightly.

Reported-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>