powerpc/stacktrace: Fix arch_stack_walk_reliable()

The changes to copy_thread() made in commit eed7c420aac7 ("powerpc:
copy_thread differentiate kthreads and user mode threads") inadvertently
broke arch_stack_walk_reliable() because it has knowledge of the stack
layout.

Fix it by changing the condition to match the new logic in
copy_thread(). The changes make the comments about the stack layout
incorrect, rather than rephrasing them just refer the reader to
copy_thread().

Also the comment about the stack backchain is no longer true, since
commit edbd0387f324 ("powerpc: copy_thread add a back chain to the
switch stack frame"), so remove that as well.

Fixes: eed7c420aac7 ("powerpc: copy_thread differentiate kthreads and user mode threads")
Reported-by: Joe Lawrence <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://msgid.link/[email protected]

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