raid5: don't duplicate code for different paths in handle_stripe

As we can see, R5_LOCKED is set and s.locked is increased whether
R5_ReWrite is set or not, so move it to common path.

Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Signed-off-by: Song Liu <songliubraving@fb.com>