mempool: Add mempool_init()/mempool_exit()

Allows mempools to be embedded in other structs, getting rid of a
pointer indirection from allocation fastpaths.

mempool_exit() is safe to call on an uninitialized but zeroed mempool.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>