fstests: fix argument passing to _run_fsstress and _run_fsstress_bg

fstests: fix argument passing to _run_fsstress and _run_fsstress_bg

After commit 8973af00ec21 ("fstests: cleanup fsstress process management")
test cases btrfs/007 and btrfs/284 started to fail. For example:

  $ ./check btrfs/007
  FSTYP         -- btrfs
  PLATFORM      -- Linux/x86_64 debian0 6.13.0-rc1-btrfs-next-181+ #1 SMP PREEMPT_DYNAMIC Tue Dec  3 13:03:23 WET 2024
  MKFS_OPTIONS  -- /dev/sdc
  MOUNT_OPTIONS -- /dev/sdc /home/fdmanana/btrfs-tests/scratch_1

  btrfs/007 1s ... [failed, exit status 1]- output mismatch (see /home/fdmanana/git/hub/xfstests/results//btrfs/007.out.bad)
      --- tests/btrfs/007.out	2020-06-10 19:29:03.810518987 +0100
      +++ /home/fdmanana/git/hub/xfstests/results//btrfs/007.out.bad	2024-12-10 16:09:56.345937619 +0000
      @@ -1,3 +1,4 @@
       QA output created by 007
       *** test send / receive
      -*** done
      +failed: '2097152000 200'
      +(see /home/fdmanana/git/hub/xfstests/results//btrfs/007.full for details)
      ...
      (Run 'diff -u /home/fdmanana/git/hub/xfstests/tests/btrfs/007.out /home/fdmanana/git/hub/xfstests/results//btrfs/007.out.bad'  to see the entire diff)
  Ran: btrfs/007

The problem comes from _run_fsstress and _run_fsstress_bg using $*, which
splits the string argument for the -x command used by btrfs/007, so that
fsstress gets the argument for -x as simply:

   "btrfs"

Instead of:

   "btrfs subvolume snapshot -r $SCRATCH_MNT $SCRATCH_MNT/base"

Fix this by using "$@" instead of $*.

Fixes: 8973af00ec21 ("fstests: cleanup fsstress process management")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>