readers/upgrading_consumer: workaround for aarch64 miscompilation
readers/upgrading_consumer: workaround for aarch64 miscompilation

On aarch64 the `std::move(mf)` seems to be reordered w.r.t.
`flush_tombstones()` in certain circumstances. These circumstances
are not clear yet, but while further investigation happens, this patch
makes the tests pass on aarch64, unclogging the promotion pipeline.

Refs: #10248
Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <20220321122209.71685-1-a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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