ide: Add ide_get_paired_drive() helper

patch 1b678347 in mainline.

This adds a helper to get to the "other" drive on a pair connected
to a given hwif.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Andrew Morton <akpm@osdl.org>
Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/linux/ide.h

@@ -1437,4 +1437,11 @@ static inline int hwif_to_node(ide_hwif_t *hwif)
 	return dev ? pcibus_to_node(dev->bus) : -1;
 }
 
+static inline ide_drive_t *ide_get_paired_drive(ide_drive_t *drive)
+{
+	ide_hwif_t *hwif	= HWIF(drive);
+
+	return &hwif->drives[(drive->dn ^ 1) & 1];
+}
+
 #endif /* _IDE_H */