generic: test race when checking i_size on direct i/o read

generic: test race when checking i_size on direct i/o read

In this commit a new test case is added to test that i_size races
don't occur under dio reads/writes.  We add a program in /src dir,
which has a writer to issue some append dio writes.  Meanwhile it
has a reader in this test to do some dio reads.  As we expect,
reader should read nothing or data with 'a'. But it might read some
data with '0'.

The bug can be reproduced by this test case [1].

1.  http://patchwork.ozlabs.org/patch/311761/

This ostensibly tests commit:
9fe55eea7 Fix race when checking i_size on direct i/o read

Update by Eric Sandeen:
- update to recent xfstests
- update commit log

Update by Eryu Guan:
- add aio-dio support to the test and add 'aio' group
- add ability to test different alignments
- move test from src/ to src/aio-dio-regress/
- add .gitignore entry
- rebase against latest xfstests with various minor fixes & cleanups
- update commit log

Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>