x86/traps: Use current_top_of_stack() helper in traps.c

Use current_top_of_stack() helper in sync_regs() and vc_switch_off_ist()
instead of open-coding the reading of  the top_of_stack percpu variable
explicitly.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20231204210320.114429-2-ubizjak@gmail.com