x86/xen/64, x86/entry/64: Clean up SP code in cpu_initialize_context()

x86/xen/64, x86/entry/64: Clean up SP code in cpu_initialize_context()

commit f16b3da1dc936c0f8121741d0a1731bf242f2f56 upstream.

I'm removing thread_struct::sp0, and Xen's usage of it is slightly
dubious and unnecessary.  Use appropriate helpers instead.

While we're at at, reorder the code slightly to make it more obvious
what's going on.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/d5b9a3da2b47c68325bd2bbe8f82d9554dee0d0f.1509609304.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>