replace_delta(): fix error message

This is an addition to commit 1275db4.

Before:
Error in replace_delta (ploop.c:1239): Invalid level 126 specified,
allowed values are 0 to 126

After:
Error in replace_delta (ploop.c:1239): Invalid level 126 specified,
allowed values are 0 to 125

Signed-off-by: Kir Kolyshkin <kir@openvz.org>