xfs: fix blocktrash fuzzers

xfs: fix blocktrash fuzzers

The blocktrash fuzz tests for xfs will try to mount and write to the
filesystem after corrupting it.  However, the mount may not necessarily
succeed, in which case we must not write junk to the root filesystem.
Use the new _try_scratch_mount to guard against that.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>