xfstests generic 310: check concurrently of readdir and read on the same directory fd

xfstests generic 310: check concurrently of readdir and read on the same directory fd

Check if there are two threads,one keeps calling read() or lseek(), and
the other calling readdir(), both on the same directory fd.

Based on a testcase from Li Zefan <lizefan@huawei.com>.
http://marc.info/?l=linux-kernel&m=136123703211869&w=2

Signed-off-by: Zhao Hongjiang <zhaohongjiang@huawei.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
[rjohnston@sgi.com minor white space fixes and test renumbering]
Signed-off-by: Rich Johnston <rjohnston@sgi.com>