filter: optimize accesses to ancillary data

We can translate pseudo load instructions at filter check time to
dedicated instructions to speed up filtering and avoid one switch().
libpcap currently uses SKF_AD_PROTOCOL, but custom filters probably use
other ancillary accesses.

Note : I made the assertion that ancillary data was always accessed with
BPF_LD|BPF_?|BPF_ABS instructions, not with BPF_LD|BPF_?|BPF_IND ones
(offset given by K constant, not by K + X register)

On x86_64, this saves a few bytes of text :

# size net/core/filter.o.*
   text	   data	    bss	    dec	    hex	filename
   4864	      0	      0	   4864	   1300	net/core/filter.o.new
   4944	      0	      0	   4944	   1350	net/core/filter.o.old

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>