ip: don't export useless error counters for ip6 rewrite

the error node is set to ip6_input in the inline funcition
associated with ip6_rewrite. Thus, error counters defined
for node ip6 rewrite are never used.

Type: fix
Change-Id: Id6bef633928b0fff9069498c2e39e9f5bea2cf9b
Signed-off-by: Arthur de Kerhor <arthurdekerhor@gmail.com>