x86/bpf: Fix BPF percpu accesses

Due to this recent commit in the x86 tree:

  9d7de2aa ("Use relative percpu offsets")

percpu addresses went from positive offsets from the GSBASE to negative
kernel virtual addresses.  The BPF verifier has an optimization for
x86-64 that loads the address of cpu_number into a register, but was only
doing a 32-bit load which truncates negative addresses.

Change it to a 64-bit load so that the address is properly sign-extended.

Fixes: 9d7de2aa ("Use relative percpu offsets")
Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250227195302.1667654-1-brgerst@gmail.com