generic/251: use sentinel files to kill the fstrim loop

generic/251: use sentinel files to kill the fstrim loop

Apparently the subshell kill doesn't always take, and then the test runs
for hours and hours because nothing stops it.  Instead, use a sentinel
file to detect when fstrim_loop should stop execing background fstrims.

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