bpf: Fix a potential use-after-free of BTF object

bpf: Fix a potential use-after-free of BTF object

Refcounting in the check_pseudo_btf_id() function is incorrect:
the __check_pseudo_btf_id() function might get called with a zero
refcounted btf. Fix this, and patch related code accordingly.

v3: rephrase a comment (AI)
v2: fix a refcount leak introduced in v1 (AI)

Reported-by: syzbot+5a0f1995634f7c1dadbf@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5a0f1995634f7c1dadbf
Fixes: 76145f725532 ("bpf: Refactor check_pseudo_btf_id")
Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
Link: https://lore.kernel.org/r/20260209132904.63908-1-a.s.protopopov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>