netfilter: nf_log: move log buffering to core logging

This patch moves Eric Dumazet's log buffer implementation from the
xt_log.h header file to the core net/netfilter/nf_log.c. This also
includes the renaming of the structure and functions to avoid possible
undesired namespace clashes.

This change allows us to use it from the arp and bridge packet logging
implementation in follow up patches.