BUG/MEDIUM: net-helper: Adjust sample size capacity after pointer shift

BUG/MEDIUM: net-helper: Adjust sample size capacity after pointer shift

eth.data, eth.src and ip.data converters are concerned. On success, the area
pointer of the sample buffer is moved forward without updating the buffer
size accordingly. If these converters are followed by another one relying on
the buffer size to do some operations on the buffer area, this could lead to
a buffer overflow.

Thanks to Charles Vosburgh <theminershive@gmail.com> for reporting this and
providing the fix.

This patch must be backported to 3.4.