xfs: only look at the fork format in xfs_idestroy_fork

Stop using the XFS_IFEXTENTS flag, and instead switch on the fork format
in xfs_idestroy_fork to decide how to cleanup.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>