[NETFILTER]: amanda helper: convert to textsearch infrastructure

When a port number within a packet is replaced by a differently sized
number only the packet is resized, but not the copy of the data.
Following port numbers are rewritten based on their offsets within
the copy, leading to packet corruption.

Convert the amanda helper to the textsearch infrastructure to avoid
the copy entirely.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>