Regression test portability: truncate

Use truncate -s 1M instead of truncate --size=1m for portability sake

BUG: 1129939
Change-Id: I5bf6ca1f9bb4fa3c91796a659a06bf368776b3e5
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8894
Reviewed-by: Xavier Hernandez <xhernandez@datalab.es>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Harshavardhana <harsha@harshavardhana.net>