selftests/bpf: Fix verifier_bpf_fastcall test

selftests/bpf: Fix verifier_bpf_fastcall test

Commit [1] moves percpu data on x86 from address 0x000... to address
0xfff...

Before [1]:

159020: 0000000000030700     0 OBJECT  GLOBAL DEFAULT   23 pcpu_hot

After [1]:

152602: ffffffff83a3e034     4 OBJECT  GLOBAL DEFAULT   35 pcpu_hot

As a result, verifier_bpf_fastcall tests should now expect a negative
value for pcpu_hot, IOW, the disassemble should show "r=" instead of
"w=".

Fix this in the test.

Note that, a later change created a new variable "cpu_number" for
bpf_get_smp_processor_id() [2]. The inlining logic is updated properly
as part of this change, so there is no need to fix anything on the
kernel side.

[1] commit 9d7de2aa8b41 ("x86/percpu/64: Use relative percpu offsets")
[2] commit 01c7bc5198e9 ("x86/smp: Move cpu number to percpu hot section")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20250328193124.808784-1-song@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>