mount: Invert check for shared mounts check
mount: Invert check for shared mounts check

Introduced by eb214be2, the empty mnt_share list cannot
produce the list_first_entry element :)

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>
Acked-by: Andrew Vagin <74c05afa918858348d6db8037d82fa8124870113@parallels.com>

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