xfstests shared/298: various fixes

xfstests shared/298: various fixes

fix shared/298:

* don't include common/config, not needed and breaks
  stuff when $SCRATCH_DEV_POOL is defined:
    Error: $SCRATCH_DEV should be unset when $SCRATCH_DEV_POOL is set
* make sure xfs_io has fiemap, we'll need it
* add -F to the xfs_io invocation, again, for use on
  old xfsprogs on non-xfs filesystems
* ignore ENOSPC errors from "garbage" loop; the only goal
  is to fill it, ENOSPC doesn't matter.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>