frag6.c: move variables and sysctls into local file

Move the sysctls and the related variables only used in frag6.c
into the file and out of in6_proto.c. That way everything belonging
together is in one place.

Sort the variables into global and per-vnet scopes and make
them static. No longer export the (helper) function
frag6_set_bucketsize() now also file-local only.

Should be no functional changes, only reduced public KPI/KBI surface.

MFC after: 3 months
Sponsored by: Netflix