[NET] NEIGHBOUR: Extract hash/lookup functions for pneigh entries.

Extract hash function for pneigh entries from pneigh_lookup(),
__pneigh_lookup() and pneigh_delete() as pneigh_hash().
Extract core of pneigh_lookup() and __pneigh_lookup() as
__pneigh_lookup_1().

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>