x86/softirq/64: Inline do_softirq_own_stack()

There is no reason to have this as a seperate function for a single caller.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210210002513.382806685@linutronix.de