osd: Write non-zero data as part of osd benchmark test.

osd: Write non-zero data as part of osd benchmark test.

An optimization (see PR: https://github.com/ceph/ceph/pull/43337) was made
in BlueStore to avoid writing bufferlists made up of zeros. The osd
benchmark used zero filled bufferlists and this resulted in inflated osd
benchmark results.

This issue is fixed by using bufferlists filled with non-zero values.

Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>
Fixes: https://tracker.ceph.com/issues/54364
(cherry picked from commit 09f94ace863255a7dd7075e269f8d7d63a398495)