In preparation to enable -Wimplicit-fallthrough for Clang, fix a
warning by explicitly adding a fallthrough; statement.
Notice that this seems to be a Duff device for performance[1]. So,
although the code looks a bit _funny_, I didn't want to refactor
or modify it beyond merely adding a fallthrough marking, which
might be the least disruptive way to fix this issue.
[1] https://www.drdobbs.com/a-reusable-duff-device/184406208
Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
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