xfs/117: fix fragility in this fuzz test

This fuzz test has some fragility problems -- it doesn't do anything to
guarantee that the inodes that it checks for EFSCORRUPTED are the same
ones that it fuzzed, and it doesn't explicitly try to avoid victimizing
inodes in the same chunk as the root directory.  As a result, this test
fails annoyingly frequently.

Fix both of these problems and get rid of the confusingly named TESTDIR
variable.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>