conntrack: fix ftp ipv4 address substitution.

conntrack: fix ftp ipv4 address substitution.

When replacing the ipv4 address in repl_ftp_v4_addr(), the remaining size
was incorrectly calculated which could lead to the wrong replacement
adjustment.

This goes unnoticed most of the time, unless you choose carefully your
initial and replacement addresses.

Example fail address combination with 10.1.1.200 DNAT'd to 10.1.100.1.

Fix this by doing something similar to V6 and also splicing out common
code for better coverage and maintainability.

A test is updated to exercise different initial and replacement addresses
and another test is added.

Fixes: bd5e81a0e596 ("Userspace Datapath: Add ALG infra and FTP.")
Reported-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>