generic: test deadlock on O_DIRECT|O_DSYNC

generic: test deadlock on O_DIRECT|O_DSYNC

We had a problem recently where btrfs would deadlock with
O_DIRECT|O_DSYNC because of an unexpected dependency on ->fsync in
iomap.  This was only caught by chance with aiostress, because weirdly
we don't actually test this particular configuration anywhere in
xfstests.  Fix this by adding a basic test that just does
O_DIRECT|O_DSYNC writes.  With this test the box deadlocks right away
with Btrfs, which would have been helpful in finding this issue before
the patches were merged.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>