very probably the reason is a mistmatch between cloop and insmod
not compiled with the same kernel_version
To be sure you should do this before compiling cloop
mv /usr/include/linux /usr/include/linux_old
ln -s /usr/src/linux/include/linux /usr/include/linux
and make sure that /usr/src/linux pointing to your kernel you are using ,for example
ln -s /usr/src/linux-2.6.5 /usr/src/linux
compile cloop then,and try insmod cloop.o
hth