raid5-ppl: silence a misleading warning message

The "need_cache_flush" variable is never set to false.  When the
variable is true that means we print a warning message at the end of
the function.

Fixes: 3418d036 ("raid5-ppl: Partial Parity Log write logging implementation")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>