raid5-cache: add another check conditon before replaying one stripe

New stripe that was just allocated has no STRIPE_R5C_CACHING state too,
add this check condition could avoid unnecessary replaying for empty stripe.

r5l_recovery_replay_one_stripe would reset stripe for any case, delete it
to make code more clean.

Signed-off-by: Zhengyuan Liu <liuzhengyuan@kylinos.cn>
Signed-off-by: Shaohua Li <shli@fb.com>