LU-9516 tests: fix sanity test_24v

LU-9516 tests: fix sanity test_24v

The "lfs getdirstripe -c" command will return stripes=0 for
unstriped directories.  Handle this when calculating free_inodes
to avoid creating zero files for this test.

Speed cleanup of test_24v and other users of simple_cleanup_common()
by using unlinkmany to delete files if the file count is provided.

Use stack_trap consistently and don't do both manual and exit cleanup.

Test-Parameters: trivial testlist=sanity env=ONLY=24v
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I25105a5d0ab719d41bf41cff0aaea6d00a9c4059
Reviewed-on: https://review.whamcloud.com/45172
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>