bpf, sockmap: Remove bucket->lock from sock_{hash|map}_free

The bucket->lock is not needed in the sock_hash_free and sock_map_free
calls, in fact it is causing a splat due to being inside rcu block.

| BUG: sleeping function called from invalid context at net/core/sock.c:2935
| in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 62, name: kworker/0:1
| 3 locks held by kworker/0:1/62:
|  #0: ffff88813b019748 ((wq_completion)events){+.+.}, at: process_one_work+0x1d7/0x5e0
|  #1: ffffc900000abe50 ((work_completion)(&map->work)){+.+.}, at: process_one_work+0x1d7/0x5e0
|  #2: ffff8881381f6df8 (&stab->lock){+...}, at: sock_map_free+0x26/0x180
| CPU: 0 PID: 62 Comm: kworker/0:1 Not tainted 5.5.0-04008-g7b083332376e #454
| Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_073836-buildvm-ppc64le-16.ppc.fedoraproject.org-3.fc31 04/01/2014
| Workqueue: events bpf_map_free_deferred
| Call Trace:
|  dump_stack+0x71/0xa0
|  ___might_sleep.cold+0xa6/0xb6
|  lock_sock_nested+0x28/0x90
|  sock_map_free+0x5f/0x180
|  bpf_map_free_deferred+0x58/0x80
|  process_one_work+0x260/0x5e0
|  worker_thread+0x4d/0x3e0
|  kthread+0x108/0x140
|  ? process_one_work+0x5e0/0x5e0
|  ? kthread_park+0x90/0x90
|  ret_from_fork+0x3a/0x50

The reason we have stab->lock and bucket->locks in sockmap code is to
handle checking EEXIST in update/delete cases. We need to be careful during
an update operation that we check for EEXIST and we need to ensure that the
psock object is not in some partial state of removal/insertion while we do
this. So both map_update_common and sock_map_delete need to guard from being
run together potentially deleting an entry we are checking, etc. But by the
time we get to the tear-down code in sock_{ma[|hash}_free we have already
disconnected the map and we just did synchronize_rcu() in the line above so
no updates/deletes should be in flight. Because of this we can drop the
bucket locks from the map free'ing code, noting no update/deletes can be
in-flight.

Fixes: 604326b4 ("bpf, sockmap: convert to generic sk_msg interface")
Reported-by: Jakub Sitnicki <jakub@cloudflare.com>
Suggested-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/158385850787.30597.8346421465837046618.stgit@john-Precision-5820-Tower