LU-1538 tests: fix test cases when OST is full

LU-1538 tests: fix test cases when OST is full

In sanity.sh test_101d() the test didn't check if "dd" failed to write
the full file size, and produced an confusing error about readahead
performance.

In sanityn.sh test_36() it also didn't check if "dd" failed to write
the full file size, and then multiop read was stuck in a loop of zero
length reads forever.  Fix both "dd" error checking, and multiop.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ic4d5ec90d77b1a9302d3e8f128f292b3765611d7
Reviewed-on: http://review.whamcloud.com/4265
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Li Wei <liwei@whamcloud.com>
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>