fstests: test dirty pipe vulnerability issue of CVE-2022-0847

fstests: test dirty pipe vulnerability issue of CVE-2022-0847

Test for the Dirty Pipe vulnerability (CVE-2022-0847) caused by an
uninitialized  "pipe_buffer.flags" variable. The bug cause a file
can be overwritten even if a user/process is not permitted to write
it. It's fixed by 9d2231c5d74e ("lib/iov_iter: initialize "flags" in
new pipe_buffer").

Cc: Max Kellermann <max.kellermann@ionos.com>
Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>