selftests: splice_read: Fix sysfs read cases

sysfs now supports splice_* operations with

 commit f2d6c270 ("kernfs: wire up ->splice_read and ->splice_write")

Update the selftests to expect success instead of failure.

Cc: Shuah Khan <shuah@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Siddharth Gupta <sidgup@codeaurora.org
Reported-by: Dilip Kota <dilip.kota@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>