x86/stacktrace: Fix reliable check for empty user task stacks

If a user task's stack is empty, or if it only has user regs, ORC
reports it as a reliable empty stack.  But arch_stack_walk_reliable()
incorrectly treats it as unreliable.

That happens because the only success path for user tasks is inside the
loop, which only iterates on non-empty stacks.  Generally, a user task
must end in a user regs frame, but an empty stack is an exception to
that rule.

Thanks to commit 71c95825 ("x86/unwind/orc: Fix error handling in
__unwind_start()"), unwind_start() now sets state->error appropriately.
So now for both ORC and FP unwinders, unwind_done() and !unwind_error()
always means the end of the stack was successfully reached.  So the
success path for kthreads is no longer needed -- it can also be used for
empty user tasks.

Reported-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Link: https://lkml.kernel.org/r/f136a4e5f019219cbc4f4da33b30c2f44fa65b84.1594994374.git.jpoimboe@redhat.com