xfs: fix UAF in xchk_btree_check_block_owner

xfs: fix UAF in xchk_btree_check_block_owner

We cannot dereference bs->cur when trying to determine if bs->cur
aliases bs->sc->sa.{bno,rmap}_cur after the latter has been freed.
Fix this by sampling before type before any freeing could happen.
The correct temporal ordering was broken when we removed xfs_btnum_t.

Cc: r772577952@gmail.com
Cc: <stable@vger.kernel.org> # v6.9
Fixes: ec793e690f801d ("xfs: remove xfs_btnum_t")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Jiaming Zhang <r772577952@gmail.com>