fstests: explicitly specify xattr namespace

fstests: explicitly specify xattr namespace

Explicitly specify the xattr namespace required for tests.
This allows tests to be skipped correctly for filesystems
that don't support all xattr namespaces.

This changes all tests that require anything other than
the "user" xattr namespace. When called without arguments
as before, _require_attrs() still defaults to the "user"
namespace, so those tests do not need to be changed.

Signed-off-by: Frank van der Linden <fllinden@amazon.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>