We are currently wasting ~2MiB on the bio pools. This is ok on systems
with plenty of ram, but it's too much for a 16mb system for instance.

This patch scales the bio_vec mempool sizes a bit. The logic is mainly:

+       megabytes = nr_free_pages() >> (20 - PAGE_SHIFT);
+       if (megabytes <= 16)
+               scale = 0;
+       else if (megabytes <= 32)
+               scale = 1;
+       else if (megabytes <= 64)
+               scale = 2;
+       else if (megabytes <= 96)
+               scale = 3;
+       else if (megabytes <= 128)
+               scale = 4;

and then for mempool setup:

+               if (i >= scale)
+                       pool_entries >>= 1;
+
+               bp->pool = mempool_create(pool_entries, slab_pool_alloc,
                                        slab_pool_free, bp->slab);

So we allocate less and less entries for the bigger sized pools. It
doesn't make too much sense to fill the memory with sg tables for 256
page entries on a 16mb system.

In addition, we select a starting nr_pool_entries point, based on amount
of ram as well:

+       pool_entries = megabytes * 2;
+       if (pool_entries > 256)
+               pool_entries = 256;

The end-result is that on a 128mb system, it looks like:

BIO: pool of 256 setup, 14Kb (56 bytes/bio)
biovec pool[0]:   1 bvecs: 244 entries (12 bytes)
biovec pool[1]:   4 bvecs: 244 entries (48 bytes)
biovec pool[2]:  16 bvecs: 244 entries (192 bytes)
biovec pool[3]:  64 bvecs: 244 entries (768 bytes)
biovec pool[4]: 128 bvecs: 122 entries (1536 bytes)
biovec pool[5]: 256 bvecs:  61 entries (3072 bytes)

ie a total of ~620KiB used. Booting with mem=32m gives us:

BIO: pool of 256 setup, 14Kb (56 bytes/bio)
biovec pool[0]:   1 bvecs:  56 entries (12 bytes)
biovec pool[1]:   4 bvecs:  28 entries (48 bytes)
biovec pool[2]:  16 bvecs:  14 entries (192 bytes)
biovec pool[3]:  64 bvecs:   7 entries (768 bytes)
biovec pool[4]: 128 bvecs:   3 entries (1536 bytes)
biovec pool[5]: 256 bvecs:   1 entries (3072 bytes)

ie a total of ~31KiB. Booting with 512mb makes it:

BIO: pool of 256 setup, 14Kb (56 bytes/bio)
biovec pool[0]:   1 bvecs: 256 entries (12 bytes)
biovec pool[1]:   4 bvecs: 256 entries (48 bytes)
biovec pool[2]:  16 bvecs: 256 entries (192 bytes)
biovec pool[3]:  64 bvecs: 256 entries (768 bytes)
biovec pool[4]: 128 bvecs: 256 entries (1536 bytes)
biovec pool[5]: 256 bvecs: 256 entries (3072 bytes)

which is the same as before. The cut-off point is somewhere a bit over
256mb. Andrew suggested we may want to 'cheat' a bit here, and leave the
busy pools alone. We know that mpage is going to be heavy on the 16
entry pool, so it migh make sense to make such a pool and not scale
that. We can deal with that later, though.