libceph: format class info at init time

An object class method is formatted using a pagelist which contains
the class name, the method name, and the data concatenated into an
osd request's outbound data.

Currently when a class op is initialized in osd_req_op_cls_init(),
the lengths of and pointers to these three items are recorded.
Later, when the op is getting formatted into the request message, a
new pagelist is created and that is when these items get copied into
the pagelist.

This patch makes it so the pagelist to hold these items is created
when the op is initialized instead.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>