x86/entry: Mark check_user_regs() noinstr

It's called from the non-instrumentable section.

Fixes: c9c26150 ("x86/entry: Assert that syscalls are on the right stack")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200708192934.191497962@linutronix.de