log: add pr_err_once
log: add pr_err_once

This function prints an error message only once.
[xemul: change ({}) to do {} while (0)]

Signed-off-by: Andrey Vagin <74c05afa918858348d6db8037d82fa8124870113@openvz.org>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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