BUG/MEDIUM: http_fetch: fix "req.body_len" and "req.body_size" fetch methods in HTX...

BUG/MEDIUM: http_fetch: fix "req.body_len" and "req.body_size" fetch methods in HTX mode

When in HTX mode, in functions smp_fetch_body_len() and
smp_fetch_body_size() we were subtracting the size of each header block
from the total size htx->data to calculate the size of body, and that
could result in wrong calculated value.

To avoid this, we now loop on blocks to sum up the size of only those
that are of type HTX_BLK_DATA.

This patch must be backported to 1.9.