libceph: factor out __prepare_write_connect()

commit c0f56b483aa09c99bfe97409a43ad786f33b8a5a upstream.

Will be used for sending ceph_msg_connect with an updated authorizer,
after the server challenges the initial authorizer.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Sage Weil <sage@redhat.com>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>