vmalloc: use kzalloc() instead of alloc_bootmem()

We can call vmalloc_init() after kmem_cache_init() and use kzalloc() instead of
the bootmem allocator when initializing vmalloc data structures.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Nick Piggin <npiggin@suse.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>