[PATCH] mempool: add page allocator

This will be used by the next patch in the series to replace duplicate
mempool-backed page allocators in 2 places in the kernel.  It is also likely
that there will be more users in the future.

Signed-off-by: Matthew Dobson <colpatch@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>