netfilter: nf_nat_sip: fix via header translation with multiple parameters

Via-headers are parsed beginning at the first character after the Via-address.
When the address is translated first and its length decreases, the offset to
start parsing at is incorrect and header parameters might be missed.

Update the offset after translating the Via-address to fix this.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>