md/raid5-cache: stripe reclaim only counts valid stripes

When log space is tight, we try to reclaim stripes from log head. There
are stripes which can't be reclaimed right now if some conditions are
met. We skip such stripes but accidentally count them, which might cause
no stripes are claimed. Fixing this by only counting valid stripes.

Cc: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>