LU-1028 tests: regrab some grant after oos test

LU-1028 tests: regrab some grant after oos test

After out-of-space test, the client can see a state that avail_grant,
dirty_bytes and lost_grant are zero. In this case, page_mkwrite()
won't work because it can't allocate any grant and this affects fsx
test.

This problem is unlikely to be seen in real world. Therefore, we
wrote a small piece of data to each OSTs through each client mount
to reallocate grant.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I1d71af0a80491855cf6328a407dd417954c4e60e
Reviewed-on: http://review.whamcloud.com/3711
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: wangdi <di.wang@whamcloud.com>