mkfs: warning about misaligned AGs and RAID stripes is not an error

mkfs: warning about misaligned AGs and RAID stripes is not an error

I've noticed a fair number of fstests failures when we create a scratch
device on a RAID device and the test specifies an explicit AG count or
AG size:

--- /tmp/fstests/tests/xfs/042.out	2022-09-01 15:09:11.484679979 -0700
+++ /var/tmp/fstests/xfs/042.out.bad	2023-04-25 19:59:04.040000000 -0700
@@ -1,5 +1,8 @@
 QA output created by 042
-Make a 96 megabyte filesystem on SCRATCH_DEV and mount... done
+Make a 96 megabyte filesystem on SCRATCH_DEV and mount... Warning: AG size is a multiple of stripe width.  This can cause performance
+problems by aligning all AGs on the same disk.  To avoid this, run mkfs with
+an AG size that is one stripe unit smaller or larger, for example 8160.
+done

Emitting this warning on stderr is silly -- nothing has failed, and we
aren't going to abort the format either.  Send the warning to stdout.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>