md: Simplify code with existing definition RESYNC_SECTORS in raid10.c

#define RESYNC_SECTORS (RESYNC_BLOCK_SIZE >> 9)

"RESYNC_BLOCK_SIZE/512" is equal to "RESYNC_BLOCK_SIZE >> 9", replace it
with RESYNC_SECTORS.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Song Liu <songliubraving@fb.com>