selftests: splice: Adjust for handler fallback removal

Some pseudo-filesystems do not have an explicit splice fops since adding
commit 36e2c742 ("fs: don't allow splice read/write without explicit ops"),
and now will reject attempts to use splice() in those filesystem paths.

Reported-by: kernel test robot <rong.a.chen@intel.com>
Link: https://lore.kernel.org/lkml/202009181443.C2179FB@keescook/


Fixes: 36e2c742 ("fs: don't allow splice read/write without explicit ops")
Cc: Christoph Hellwig <hch@lst.de>
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>