tests/ec: Avoid spurious umount failures

Change-Id: Id22d64a95adf3666a5e4208f87f9a6d91c40b267
BUG: 1092850
Signed-off-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-on: http://review.gluster.org/8694
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Emmanuel Dreyfus <manu@netbsd.org>
Tested-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>