LU-15981 tests: add missing close in cascading_rw

LU-15981 tests: add missing close in cascading_rw

ONLY=cascading_rw OSTSIZE=200000 SLOW=yes sh parallel-scale.sh used to
fail due to lack of space.

cascading_rw missed a close before unlink, so disk space got consumed
by open unlinked files.

Add the missing close().

Change-Id: I9a8b3bfb9ff00e604ae0439433599b5050191b8e
HPE-bug-id: LUS-10753
Test-Parameters: testlist=trivial testlist=parallel-scale envdefinitions=ONLY=cascading_rw
Signed-off-by: Vladimir Saveliev <vladimir.saveliev@hpe.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/47823
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>