se kmem_cache_zalloc()

Replace kmem_cache_alloc + memset(,0,) to kmem_cache_zalloc.
I think it's not harmful since @conf->slab_cache already knows
actual size of struct stripe_head.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>
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