BUG/MINOR: filters: Use filter offset to decude the amount of forwarded data

BUG/MINOR: filters: Use filter offset to decude the amount of forwarded data

When the tcp or http payload is filtered, it is important to use the filter
offset to decude the amount of forwarded data because this offset may change
during the call to the callback function. So we should not rely on a local
variable defined before this call.

For now, existing HAproxy filters don't change this offset, so this bug may only
affect external filters.

This patch must be forwarded as far as 1.9.

(cherry picked from commit c50ee0b3b4001300f47f77bbdc38b2991b76d663)
Signed-off-by: Willy Tarreau <w@1wt.eu>