bpf: Fix race between btf_put and btf_idr walk.

Florian and Eduard reported hard dead lock:
[   58.433327]  _raw_spin_lock_irqsave+0x40/0x50
[   58.433334]  btf_put+0x43/0x90
[   58.433338]  bpf_find_btf_id+0x157/0x240
[   58.433353]  btf_parse_fields+0x921/0x11c0

This happens since btf->refcount can be 1 at the time of btf_put() and
btf_put() will call btf_free_id() which will try to grab btf_idr_lock
and will dead lock.
Avoid the issue by doing btf_put() without locking.

Fixes: 3d78417 ("bpf: Add bpf_btf_find_by_name_kind() helper.")
Fixes: 1e89106 ("bpf: Add bpf_core_add_cands() and wire it into bpf_core_apply_relo_insn().")
Reported-by: Florian Westphal <fw@strlen.de>
Reported-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20230421014901.70908-1-alexei.starovoitov@gmail.com

kernel/bpf/btf.c

@@ -577,8 +577,8 @@ static s32 bpf_find_btf_id(const char *name, u32 kind, struct btf **btf_p)
 			*btf_p = btf;
 			return ret;
 		}
-		spin_lock_bh(&btf_idr_lock);
 		btf_put(btf);
+		spin_lock_bh(&btf_idr_lock);
 	}
 	spin_unlock_bh(&btf_idr_lock);
 	return ret;
@@ -8354,12 +8354,10 @@ bpf_core_find_cands(struct bpf_core_ctx *ctx, u32 local_type_id)
 		btf_get(mod_btf);
 		spin_unlock_bh(&btf_idr_lock);
 		cands = bpf_core_add_cands(cands, mod_btf, btf_nr_types(main_btf));
-		if (IS_ERR(cands)) {
-			btf_put(mod_btf);
+		btf_put(mod_btf);
+		if (IS_ERR(cands))
 			return ERR_CAST(cands);
-		}
 		spin_lock_bh(&btf_idr_lock);
-		btf_put(mod_btf);
 	}
 	spin_unlock_bh(&btf_idr_lock);
 	/* cands is a pointer to kmalloced memory here if cands->cnt > 0