bpf: Fix tr dereferencing

Fix 'tr' dereferencing bug when CONFIG_BPF_JIT is turned off.

When CONFIG_BPF_JIT is turned off, 'bpf_trampoline_get()' returns NULL,
which is same as the cases when CONFIG_BPF_JIT is turned on.

Closes: https://lore.kernel.org/r/202309131936.5Nc8eUD0-lkp@intel.com/
Fixes: f7b12b6 ("bpf: verifier: refactor check_attach_btf_id()")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Leon Hwang <hffilwlqm@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230917153846.88732-1-hffilwlqm@gmail.com

include/linux/bpf.h

@@ -1307,7 +1307,7 @@ static inline int bpf_trampoline_unlink_prog(struct bpf_tramp_link *link,
 static inline struct bpf_trampoline *bpf_trampoline_get(u64 key,
 							struct bpf_attach_target_info *tgt_info)
 {
-	return ERR_PTR(-EOPNOTSUPP);
+	return NULL;
 }
 static inline void bpf_trampoline_put(struct bpf_trampoline *tr) {}
 #define DEFINE_BPF_DISPATCHER(name)