xfs: add more attribute tree trace points.

Added when debugging recent attribute tree problems to more finely
trace code execution through the maze of twisty passages that makes
up the attr code.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>