In swapgs_restore_regs_and_return_to_usermode, after the stack is
switched to the trampoline stack, the existing UNWIND_HINT_REGS hint is
no longer valid, which can result in the following ORC unwinder warning:

  WARNING: can't dereference registers at 000000003aeb0cdd for ip swapgs_restore_regs_and_return_to_usermode+0x93/0xa0

For full correctness, we could try to add complicated unwind hints so
the unwinder could continue to find the registers, but when when it's
this close to kernel exit, unwind hints aren't really needed anymore and
it's fine to just use an empty hint which tells the unwinder to stop.

For consistency, also move the UNWIND_HINT_EMPTY in
entry_SYSCALL_64_after_hwframe to a similar location.

Fixes: 3e3b9293 ("x86/entry/64: Return to userspace from the trampoline stack")
Reported-by: NVince Weaver <vincent.weaver@maine.edu>
Reported-by: NDave Jones <dsj@fb.com>
Reported-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reported-by: NJoe Mario <jmario@redhat.com>
Reported-by: NJann Horn <jannh@google.com>
Reported-by: NLinus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: NMiroslav Benes <mbenes@suse.cz>
Signed-off-by: NJosh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: NIngo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/60ea8f562987ed2d9ace2977502fe481c0d7c9a0.1587808742.git.jpoimboe@redhat.com