libceph: add start en/decoding block helpers

Add ceph_start_encoding() and ceph_start_decoding(), the equivalent of
ENCODE_START and DECODE_START in the userspace ceph code.

This is based on a patch from Mike Christie <michaelc@cs.wisc.edu>.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>