generic: test concurrent direct IO writes and fsync using same fd

generic: test concurrent direct IO writes and fsync using same fd

Test that a program that has 2 threads using the same file descriptor and
concurrently doing direct IO writes and fsync doesn't trigger any crash
or deadlock.

This is motivated by a bug found in btrfs fixed by the following patch:

  "btrfs: fix race between direct IO write and fsync when using same fd"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>