fix #2178: endless loop on ipv6 extension headers

fix #2178: endless loop on ipv6 extension headers

increment header and decrement payload size by the extensions size. the
length calculation is different for some extensions. in our case only
IPPROTO_FRAGMENT requires a different size calculation than the rest. in
addition 'proto' is now set in the loop when advancing from an
extension header. it moves on to the next extension or protocol now
instead of looping on the same 'proto' while advancing the payload.

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>