bcache: avoid extra memory allocation from mempool c->fill_iter

Mempool c->fill_iter is used to allocate memory for struct btree_iter in
bch_btree_node_read_done() to iterate all keys of a read-in btree node.

The allocation size is defined in bch_cache_set_alloc() by,
  mempool_init_kmalloc_pool(&c->fill_iter, 1, iter_size))
where iter_size is defined by a calculation,
  (sb->bucket_size / sb->block_size + 1) * sizeof(struct btree_iter_set)

For 16bit width bucket_size the calculation is OK, but now the bucket
size is extended to 32bit, the bucket size can be 2GB. By the above
calculation, iter_size can be 2048 pages (order 11 is still accepted by
buddy allocator).

But the actual size holds the bkeys in meta data bucket is limited to
meta_bucket_pages() already, which is 16MB. By the above calculation,
if replace sb->bucket_size by meta_bucket_pages() * PAGE_SECTORS, the
result is 16 pages. This is the size large enough for the mempool
allocation to struct btree_iter.

Therefore in worst case every time mempool c->fill_iter allocates, at
most 4080 pages are wasted and won't be used. Therefore this patch uses
meta_bucket_pages() * PAGE_SECTORS to calculate the iter size in
bch_cache_set_alloc(), to avoid extra memory allocation from mempool
c->fill_iter.

Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>