xfs: cleanup xfs_idestroy_fork

Move freeing the dynamically allocated attr and COW fork, as well
as zeroing the pointers where actually needed into the callers, and
just pass the xfs_ifork structure to xfs_idestroy_fork.  Also simplify
the kmem_free calls by not checking for NULL first.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>