direct_io/dma_thread_diotest7.c: cleanup and add it to run default

This test will create 100 files(every is 12MB), so the total needed free space is
1200MB at least. If the current test temporary directory do not satisfy this
requirement, wo need to specify a big block device by running runltp with -z option.
If satisfied, we also need to check whether filesystem, which current test temporary
directory is in, supports the O_DIRECT flag. If not supported, we will also need to
have tests in a big block device specified by users.

Rename runtest/test_dma_thread_diotest7 to runtest/dma_thread_diotest and
split tests in runtest/dma_thread_diotest into separate testcases, keep one entry
per test in runtest/dma_thread_diotest. And remove test_dma_thread_diotest7.sh,
which is useless now.

Some cleanup.

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>