brd: Remove use of page->index

brd: Remove use of page->index

This debugging check will become more costly in the future when we shrink
struct page.  It has not proven to be useful, so simply remove it.

This lets us use __xa_insert instead of __xa_cmpxchg() as we no longer
need to know about the page that is currently stored in the XArray.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20240315181212.2573753-1-willy@infradead.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>