slub: zero page to fix boot crashes

Latest mmotm rarely boots if SLUB is enabled: earlyprintk=vga shows
it crashing with various backtraces.  The memset has now been removed
from kmem_cache_open(), so kmem_cache_init() needs to zero its page.
This gets SLUB booting reliably again.

Signed-off-by: Hugh Dickins <hughd@google.com>