generic/449: make the test effective against xfs

generic/449: make the test effective against xfs

Setting acls on an xfs filesystem will succeed even after running
out of space for user attributes. Use trusted attributes instead.
Also speed up the test by setting large values for the attributes.

[eguan: use perl to generate attr value, and add comments on trusted
namespace]

Signed-off-by: Ernesto A. Fernández <ernesto.mnd.fernandez@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>