generic/410: don't blow away seqres.full during test

generic/410: don't blow away seqres.full during test

Don't truncate $seqres.full every time we format a new filesystem; this
makes debugging of this weird failure:

  --- /tmp/fstests/tests/generic/410.out	2023-07-11 12:18:21.642971022 -0700
  +++ /var/tmp/fstests/generic/410.out.bad	2023-11-29 01:13:00.020000000 -0800
  @@ -107,6 +107,9 @@ mpB/dir SCRATCH_DEV
   mpC SCRATCH_DEV
   mpC/dir SCRATCH_DEV
   ======
  +mkdir: cannot create directory '/mnt/410/3871733_mpA': File exists
  +mkdir: cannot create directory '/mnt/410/3871733_mpB': File exists
  +mkdir: cannot create directory '/mnt/410/3871733_mpC': File exists
   make-shared a slave shared mount
   before make-shared run on slave shared
   ------

nearly impossible.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>