LU-1689 tests: fix mount during e2fsck test

LU-1689 tests: fix mount during e2fsck test

The current mmp test 8 (mount during e2fsck) has two time issues:
1) the mount operation may start before e2fsck
2) the e2fsck operation may stop before mount

This patch fixes the above issues by providing enough time for e2fsck
operation to be started before mount operation, and setting the
superblock free_blocks_count field with 0 to force e2fsck checking
the Lustre server target device, which provides enough time for
the mount operation to be started during the e2fsck operation.

Test-Parameters: testlist=mmp
Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: I5be2b84f063a0db386a8d9d48db53c00ebd77864
Reviewed-on: http://review.whamcloud.com/3643
Reviewed-by: Li Wei <liwei@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>