Regression test portability: dd usage

NetBSD, FreeBSD, and MacOS X dd(1) bs argument uses m for megabyte, while
Linux uses M. Use bs=1024k instead of bs=1M for better compatibility.

BUG: 764655
Change-Id: I603f57adbc9b31f6d634b918726437fbfce42e03
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8278
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-by: Justin Clift <justin@gluster.org>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>