fstests: xfs/227 is really slow

fstests: xfs/227 is really slow

The slowest test tto run on my test VMs is xfs/227:

...
xfs/227 826
...

It is doing nested iteration on created filesets that are explicitly
defined, so separate the inner loop filesets and run the outer loops
in parallel.

Also reduce the number of times we have to execute setfattr and
xfs_io to once per created file instead of once per xattr/extent
count per file.

The result is test runtime reduction of ~60%.

....
xfs/227 336
....

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Zorro lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>