MRP-1658 ldiskfs: skip checking in readonly mode

During umount ext4_mb_release_inode_pa()->ext4_error() panic, cause
memory bitmap and on-disk bitmap are different. This happened because
umount use Lustre readonly mode.
ext4_error() function could panic on errors if no MS_RDONLY.
But for the Lustre, readonly mode should be checked with
dev_check_rdonly() function. This patch skip total check at
ext4_mb_release_inode_pa() for readonly, like it was before,
but slightly early. So ext4_error() could not be called.

Signed-off-by: Alexander Boyko <alexander_boyko@xyratex.com>
Change-Id: I3014e47aefb50f479fa85fc348f251919236522f
Reviewed-on: http://morpheus.xyus.xyratex.com:8443/gerrit/1510
Tested-by: Jenkins
Reviewed-by: Andrew Perepechko <Andrew_Perepechko@xyratex.com>
Tested-by: Elena Gryaznova <Elena_Gryaznova@xyratex.com>
Reviewed-by: Alexey Lyashkov <Alexey_Lyashkov@xyratex.com>
Reviewed-by: Vitaly Fertman <Vitaly_Fertman@xyratex.com>