BUG/MEDIUM: htx: Don't count delta twice when block value is replaced

BUG/MEDIUM: htx: Don't count delta twice when block value is replaced

A regression was introduced by the commit a8887e55a ("BUG/MEDIUM: htx: Fix
function used to change part of a block value when defrag").

When a block value was replaced and a defragmentation was performed, the
delta between the old value and the new one was counted twice. htx_defrag()
already is responsible to set the new size for the HTX message. So it must
not be performed in htx_replace_blk_value().

This patch must be backported with the commit above. So theorically to all
stable versions.