LU-12446 tests: Fix use of undefined variable under sanityn.sh/16b

LU-12446 tests: Fix use of undefined variable under sanityn.sh/16b

Under sanityn.sh/16b variable STRIPE_BYTES is used to define
the block size within 'dd' command. However STRIPE_BYTES is
used undefined. This results in 'dd' command failing silently.
Although this does not affect the outcome of the test, the 'dd'
command used this way is no-op.

This patch fixes use of undefined variable under sanityn.sh:16b
by replacing variable STRIPE_BYTES with variable stripe_size.

Test-Parameters: trivial testlist=sanityn
Change-Id: I8bc947d93e339573759d5d37e800aae9bc3b4b18
Signed-off-by: Arshad Hussain <arshad.super@gmail.com>
Reviewed-on: https://review.whamcloud.com/35246
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>