md/raid10: fix set but not used variable in sync_request_write()

md/raid10: fix set but not used variable in sync_request_write()

Building with W=1 reports the following:

drivers/md/raid10.c: In function ‘sync_request_write’:
drivers/md/raid10.c:2441:21: error: variable ‘d’ set but not used [-Werror=unused-but-set-variable]
 2441 |                 int d;
      |                     ^
cc1: all warnings being treated as errors

Remove the usage of that variable.

Fixes: 752d0464b78a ("md: clean up accounting for issued sync IO")
Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/linux-raid/20250709104814.2307276-1-john.g.garry@oracle.com
Signed-off-by: Yu Kuai <yukuai@kernel.org>