generic/269,xfs/051: don't drop fsstress failures to stdout

generic/269,xfs/051: don't drop fsstress failures to stdout

Prior to commit f55e46d629, these two tests would run fsstress until it
hit a failure -- ENOSPC in the case of generic/269, and EIO in the case
of xfs/051.  These errors are expected, which was why stderr was also
redirected to /dev/null.  Commit f55e46d629 removed the stderr
redirection, which has resulted in a 100% failure rate.

Fix this regression by pushing stderr stream to $seqres.full.

Fixes: f55e46d629 ("fstests: redirect fsstress' stdout to $seqres.full instead of /dev/null")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>