Fix panic when loading libiconv.ko on sparc64.

Fix panic when loading libiconv.ko on sparc64.
It seems that this issue only become obvious when compiled with -O2
on sparc64.

Since each struct iconv_converter_class has been initialized by
DEFINE_CLASS macro, not all members of struct iconv_converter_class
has been allocated on memory and cc_link member has not been
initialized, while iconv_register_converter() wanted to access it
with TAILQ.
Now we modify KICONV_CONVERTER macro and fix this bug.

Problem reported on:	freebsd-sparc64
Pointed out by:		yongari
Discussed with:		yongari
Tested by:		yongari
MFC after:		3 days