xfs: remove __xfs_free_extent_later

xfs: remove __xfs_free_extent_later

Source kernel commit: 4c88fef3af4a51c2cdba6a28237e98da4873e8dc

xfs_free_extent_later is a trivial helper, so remove it to reduce the
amount of thinking required to understand the deferred freeing
interface.  This will make it easier to introduce automatic reaping of
speculative allocations in the next patch.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>