x86/percpu: Return correct variable from current_top_of_stack()

current_top_of_stack() should return variable from _seg_gs
qualified named address space when CONFIG_USE_X86_SEG_SUPPORT=y
is enbled.

Fixes: ed2f752e ("x86/percpu: Introduce const-qualified const_pcpu_hot to micro-optimize code generation")
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20231024142830.3226-1-ubizjak@gmail.com
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>