generic: busy loop of dd and rm test

generic: busy loop of dd and rm test

Add a case for testing whether writing failed on NO_SPACE in a busy
loop of write and delete when disk almost full.  It is a long-term
problem since very beginning in btrfs, and has been fixed by
patchset titled "btrfs: Fix no_space on dd and rm loop" from
zhaolei@cn.fujitsu.com.

Signed-off-by: Wang Yanfeng <wangyf-fnst@cn.fujitsu.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>