out_loki: avoid passing mp_sbuf twice
out_loki: avoid passing mp_sbuf twice

This is a follow on from PR #3839

That issue possibly came about because the same msgpack_sbuffer is being
passed twice in function calls:
both directly as mp_sbuf and indirectly as part of mp_pck,
leading to issues if the relationship isn't clear.

This change avoids passing mp_sbuf and instead just uses mp_pck.

Signed-off-by: Richard Burakowski <cb6c7d6aee8559f0a2383a697df350face0c1c31@gmail.com>

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