misc: allow zero duration for fsstress and fsx

misc: allow zero duration for fsstress and fsx

Occasionally the common/fuzzy fuzz test helpers manage to time
something just right such that fsx or fsstress get invoked with a zero
second duration.  It's harmless to exit immediately without doing
anything, so allow this corner case.

[zlang: No action is taken if duration is 0]

Cc: fstests@vger.kernel.org # v2023.05.01
Fixes: 3e85dd4fe4236d ("misc: add duration for long soak tests")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>