fuzzy: clear out the scratch filesystem if it's too full

fuzzy: clear out the scratch filesystem if it's too full

If the online fsck stress tests run for long enough, they'll fill up the
scratch filesystem completely.  While it is interesting to test repair
functionality on a *nearly* full filesystem undergoing a heavy workload,
a totally full filesystem is really only exercising the ENOSPC handlers
in the kernel.  That's not what we came here to test, so change the
fsstress loop to detect a nearly full filesystem and erase everything
before starting fsstress again.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>