mmc: core: don't initialize block size from ext_csd if not present

If extended CSD was not available, the eMMC driver would incorrectly
set the block size to 0, as the data_sector_size field of ext_csd
was never initialized. This issue was exposed by commit 817046ec
("block: Align max_hw_sectors to logical blocksize") which caused
max_sectors and max_hw_sectors to be set to 0 after setting the block
size to 0, resulting in a kernel panic in bio_split when attempting
to read from the device. Fix it by only reading the block size from
ext_csd if it is available.

Fixes: a5075eb9 ("mmc: block: Allow disabling 512B sector size emulation")
Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Link: https://linux-review.googlesource.com/id/If244d178da4d86b52034459438fec295b02d6e60Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20210114201405.2934886-1-pcc@google.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/queue.c

@@ -384,8 +384,10 @@ static void mmc_setup_queue(struct mmc_queue *mq, struct mmc_card *card)
 		     "merging was advertised but not possible");
 	blk_queue_max_segments(mq->queue, mmc_get_max_segments(host));
 
-	if (mmc_card_mmc(card))
+	if (mmc_card_mmc(card) && card->ext_csd.data_sector_size) {
 		block_size = card->ext_csd.data_sector_size;
+		WARN_ON(block_size != 512 && block_size != 4096);
+	}
 
 	blk_queue_logical_block_size(mq->queue, block_size);
 	/*