xfs: move (and rename) the deferred bmap-free tracepoints

Rename the deferred bmap-free to extent_free and make them only
trigger when we're really running deferred ops.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>