xfstests 311: test fsync with dm flakey V4

xfstests 311: test fsync with dm flakey V4

This test sets up a dm flakey target and then runs my fsync tester I've been
using to verify btrfs's fsync() is working properly.  It will create a dm flakey
device, mount it, run my test, make the flakey device start dropping writes, and
then unmount the fs.  Then we mount it back up and make sure the md5sums match
and then run fsck on the device to make sure we got a consistent fs.  I used the
output from a run on BTRFS since it's the only one that passes this test
properly.  I verified each test manually to make sure they were in fact valid
files.  XFS and Ext4 both fail this test in one way or another.

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Acked-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
[rjohnston@sgi.com changed syncfs() to sync() for older kernels]
Signed-off-by: Rich Johnston <rjohnston@sgi.com>