ata: libata-eh: Add ata_eh_set_lpm() WARN_ON_ONCE

link->lpm_policy is initialized to ATA_LPM_UNKNOWN in ata_eh_reset().

ata_eh_set_lpm() is then only called if
link->lpm_policy != ap->target_lpm_policy (after reset)

and then only if link->lpm_policy > ATA_LPM_MAX_POWER (before
revalidation).

This means that ata_eh_set_lpm() is currently never called with
policy == ATA_LPM_UNKNOWN.

Add a WARN_ON_ONCE so that it is more obvious from reading the code that
this function is never called with policy == ATA_LPM_UNKNOWN.

Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>

Author: floatious

drivers/ata/libata-eh.c

@@ -3442,6 +3442,13 @@ static int ata_eh_set_lpm(struct ata_link *link, enum ata_lpm_policy policy,
 	    (link->flags & ATA_LFLAG_NO_LPM) || (ap && !ap->ops->set_lpm))
 		return 0;
 
+	/*
+	 * This function currently assumes that it will never be supplied policy
+	 * ATA_LPM_UNKNOWN.
+	 */
+	if (WARN_ON_ONCE(policy == ATA_LPM_UNKNOWN))
+		return 0;
+
 	/*
 	 * DIPM is enabled only for ATA_LPM_MIN_POWER,
 	 * ATA_LPM_MIN_POWER_WITH_PARTIAL, and ATA_LPM_MED_POWER_WITH_DIPM, as