VDEV_REQ_FUA should be mapped to REQ_FUA

Pre-2.6.37 kernels support REQ_FUA in request flags, but not in BIO
flags. zvols are the only consumer of VDEV_REQ_FUA and since they are
passed requests, they should be obey the REQ_FUA flag like later
kernels. This optimization will only matter on 2.6.36 and 2.6.37 because
the zvol rework changes things to use bio, where we no longer are able
to distinguish on earlier kernels

Signed-off-by: Richard Yao <ryao@gentoo.org>

include/linux/blkdev_compat.h

@@ -458,7 +458,7 @@ bio_set_flags_failfast(struct block_device *bdev, int *flags)
 #else
 #define	VDEV_WRITE_FLUSH_FUA		WRITE_BARRIER
 #define	VDEV_REQ_FLUSH			REQ_HARDBARRIER
-#define	VDEV_REQ_FUA			REQ_HARDBARRIER
+#define	VDEV_REQ_FUA			REQ_FUA
 #endif
 
 /*