bpf: sockmap, updating the sg structure should also update curr

Curr pointer should be updated when the sg structure is shifted.

Fixes: 7246d8ed ("bpf: helper to pop data from messages")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/r/20231206232706.374377-3-john.fastabend@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>