bpf: Silence a warning in btf_type_id_size()

In the Linux kernel, the following vulnerability has been resolved:

bpf: Silence a warning in btf_type_id_size()

syzbot reported a warning in [1] with the following stacktrace:
WARNING: CPU: 0 PID: 5005 at kernel/bpf/btf.c:1988 btf_type_id_size+0x2d9/0x9d0 kernel/bpf/btf.c:1988
...
RIP: 0010:btf_type_id_size+0x2d9/0x9d0 kernel/bpf/btf.c:1988
...
Call Trace:
<TASK>
map_check_btf kernel/bpf/syscall.c:1024 [inline]
map_create+0x1157/0x1860 kernel/bpf/syscall.c:1198
__sys_bpf+0x127f/0x5420 kernel/bpf/syscall.c:5040
__do_sys_bpf kernel/bpf/syscall.c:5162 [inline]
__se_sys_bpf kernel/bpf/syscall.c:5160 [inline]
__x64_sys_bpf+0x79/0xc0 kernel/bpf/syscall.c:5160
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

With the following btf
[1] DECL_TAG 'a' type_id=4 component_idx=-1
[2] PTR '(anon)' type_id=0
[3] TYPE_TAG 'a' type_id=2
[4] VAR 'a' type_id=3, linkage=static
and when the bpf_attr.btf_key_type_id = 1 (DECL_TAG),
the following WARN_ON_ONCE in btf_type_id_size() is triggered:
if (WARN_ON_ONCE(!btf_type_is_modifier(size_type) &&
!btf_type_is_var(size_type)))
return NULL;

Note that 'return NULL' is the correct behavior as we don't want
a DECL_TAG type to be used as a btf_{key,value}_type_id even
for the case like 'DECL_TAG -> STRUCT'. So there
is no correctness issue here, we just want to silence warning.

To silence the warning, I added DECL_TAG as one of kinds in
btf_type_nosize() which will cause btf_type_id_size() returning
NULL earlier without the warning.

[1] https://lore.kernel.org/bpf/000000000000e0df8d05fc75ba86@google.com/