mmc: moxart: fix handling of sgm->consumed, otherwise WARN_ON triggers

When e.g. 8 bytes are to be read, sgm->consumed equals 8 immediately after
sg_miter_next() call. The driver then increments it as bytes are read,
so sgm->consumed becomes 16 and this warning triggers in sg_miter_stop():
WARN_ON(miter->consumed > miter->length);

WARNING: CPU: 0 PID: 28 at lib/scatterlist.c:925 sg_miter_stop+0x2c/0x10c
CPU: 0 PID: 28 Comm: kworker/0:2 Tainted: G        W          6.9.0-rc5-dirty #249
Hardware name: Generic DT based system
Workqueue: events_freezable mmc_rescan
Call trace:.
 unwind_backtrace from show_stack+0x10/0x14
 show_stack from dump_stack_lvl+0x44/0x5c
 dump_stack_lvl from __warn+0x78/0x16c
 __warn from warn_slowpath_fmt+0xb0/0x160
 warn_slowpath_fmt from sg_miter_stop+0x2c/0x10c
 sg_miter_stop from moxart_request+0xb0/0x468
 moxart_request from mmc_start_request+0x94/0xa8
 mmc_start_request from mmc_wait_for_req+0x60/0xa8
 mmc_wait_for_req from mmc_app_send_scr+0xf8/0x150
 mmc_app_send_scr from mmc_sd_setup_card+0x1c/0x420
 mmc_sd_setup_card from mmc_sd_init_card+0x12c/0x4dc
 mmc_sd_init_card from mmc_attach_sd+0xf0/0x16c
 mmc_attach_sd from mmc_rescan+0x1e0/0x298
 mmc_rescan from process_scheduled_works+0x2e4/0x4ec
 process_scheduled_works from worker_thread+0x1ec/0x24c
 worker_thread from kthread+0xd4/0xe0
 kthread from ret_from_fork+0x14/0x38

This patch adds initial zeroing of sgm->consumed. It is then incremented
as bytes are read or written.
Signed-off-by: Sergei Antonov <saproj@gmail.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Fixes: 3ee0e7c3 ("mmc: moxart-mmc: Use sg_miter for PIO")
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240422153607.963672-1-saproj@gmail.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/moxart-mmc.c

@@ -300,6 +300,7 @@ static void moxart_transfer_pio(struct moxart_host *host)
 	remain = sgm->length;
 	if (remain > host->data_len)
 		remain = host->data_len;
+	sgm->consumed = 0;
 
 	if (data->flags & MMC_DATA_WRITE) {
 		while (remain > 0) {