mempools have the wrong semantics for use by buffer_heads.  The problem
scenario:

- Process A calls mempool_alloc(), asking for a buffer_head.

- While process A sleeps, process B frees up a ton of memory.

That's it.  There is no longer any memory pressure, so nobody frees any
buffer_heads, so process A does not get woken up.  I managed to trigger this
in some testing recently.

One approach would be to use a schedule_timeout(2) in mempool_alloc().

Anyway, the importance of buffer_head allocation was lessened when swapout
stopped using them, so let's just drop the mempool out of it for now.