LU-6722 jbd: double minimum journal size for RHEL7

LU-6722 jbd: double minimum journal size for RHEL7

In RHEL7 (kernel version >= 3.10.0), the maximum journal transaction
size is reduced to half of the RHEL6 case. That may cause Lustre to
complain that the declared transaction credits too large for very
small journal device. This patch increases the minimum journal size
from 4MB to 8MB for RHEL7 case, then counteract the above limitation
about the journal transaction size.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Iec8a2c561416cb7b5acce342c8ebcb845c8d7a19
Reviewed-on: http://review.whamcloud.com/15401
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>