xsk: Create and free buffer pool independently from umem

Create and free the buffer pool independently from the umem. Move
these operations that are performed on the buffer pool from the
umem create and destroy functions to new create and destroy
functions just for the buffer pool. This so that in later commits
we can instantiate multiple buffer pools per umem when sharing a
umem between HW queues and/or devices. We also erradicate the
back pointer from the umem to the buffer pool as this will not
work when we introduce the possibility to have multiple buffer
pools per umem.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/bpf/1598603189-32145-4-git-send-email-magnus.karlsson@intel.com

include/net/xdp_sock.h

@@ -20,13 +20,12 @@ struct xdp_buff;
 struct xdp_umem {
 	struct xsk_queue *fq;
 	struct xsk_queue *cq;
-	struct xsk_buff_pool *pool;
 	u64 size;
 	u32 headroom;
 	u32 chunk_size;
+	u32 chunks;
 	struct user_struct *user;
 	refcount_t users;
-	struct work_struct work;
 	struct page **pgs;
 	u32 npgs;
 	u16 queue_id;