mnt: Factor out find-mount-by-s_dev code
mnt: Factor out find-mount-by-s_dev code

And move the 2nd piece lower to avoid fwd declaration and
keep similar calls close to each other.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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