fuzzy: allow FUZZ_REWRITE_DURATION to control fsstress runtime when fuzzing

fuzzy: allow FUZZ_REWRITE_DURATION to control fsstress runtime when fuzzing

For each iteration of the fuzz test loop, we try to correct the problem,
and then we run fsstress on the (allegedly corrected) filesystem to
check that subsequent use of the filesystem won't crash the kernel or
panic.

Now that fsstress has a --duration switch, let's add a new config
variable that people can set to constrain the amount of time that a fuzz
test run takes.

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