generic/038: set a maximum runtime on this test

generic/038: set a maximum runtime on this test

This test races multiple FITRIM calls against multiple programs creating
200k small files to ensure that there are no concurrency problems with
the allocator and the FITRIM code.  This is not necessarily quick, and
the test itself does not contain any upper bound on the runtime.  On my
system that simulates storage with DRAM this takes ~5 minutes to run; on
my cloud system with newly provided discard support, I killed the test
after 27 hours.

Constrain the runtime to about the customary 30s * TIME_FACTOR.

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