tests/block/014: ignore dd error messages

tests/block/014: ignore dd error messages

The kernel commit de3510e52b0a ("null_blk: fix command timeout
completion handling") fixed null_blk driver to report ETIMEDOUT errors
for IO operations failed with a timeout. This change causes the dd call
in block/014 case to print the following error message:

dd: error reading '/dev/nullb0': Connection timed out

The presence of this message result in a failure of the test case even
without a kernel crash or hang, which is what the block/014 case is
testing. Avoid this failure by ignoring dd error messages using a
redirection of dd stderr to /dev/null.

Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>