generic/084: use src/multi_open_unlink to replace tail command

generic/084 try to run 'tail' command, tail will use inotify.
There're some limit about the number of inotify. For example
fs.inotify.max_user_instances specifies an upper limit on
the number of inotify instances that can be created per real
user ID.

When I test on a machine with 154 cpu cores, this case run
failed, and hit many warning likes:

    +tail: inotify cannot be used, reverting to polling: Too many open files

Because the fs.inotify.max_user_instances is 128, so if we
try to tail 154 files, it will be failed.

So use src/multi_open_unlink to instead of tail will avoid
this problem.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/generic/084

@@ -69,15 +69,8 @@ _scratch_mkfs >>$seqres.full 2>&1
 _scratch_mount
 
 # create, open & unlinked files so unlinked inode list is not empty
-for i in `seq 1 $nr_cpu`; do
-	testfile=$SCRATCH_MNT/$seq.unlinked.$i
-	touch $testfile
-	tail -f $testfile &
-	tail_pids="$tail_pids $!"
-done
-# give tail some time to open the file before the file is removed
-sleep 1
-rm -f $SCRATCH_MNT/$seq.unlinked.*
+src/multi_open_unlink -f $SCRATCH_MNT/$seq.unlinked -n $nr_cpu &
+open_pid=$!
 
 # start link/unlink storm
 src=$SCRATCH_MNT/$seq.target
@@ -96,8 +89,8 @@ done &
 sleep 5
 kill $! >/dev/null 2>&1
 
-kill $tail_pids $link_pids >/dev/null 2>&1
-wait $tail_pids $link_pids
+kill $open_pid $link_pids >/dev/null 2>&1
+wait $open_pid $link_pids
 
 # all done, no oops/hang expected, _check_filesystems checks SCRATCH_DEV after test
 status=0