md: use current request time as base for ktime comparisons

Request coalescing logic uses 'prev_flush_start' as base to
compare the current request start time. 'prev_flush_start' is
updated in other context.

This patch changes this by using ktime comparison base to
'req_start' for better readability of code.

Signed-off-by: Pankaj Gupta <pankaj.gupta@cloud.ionos.com>
Signed-off-by: Song Liu <songliubraving@fb.com>