powerpc/64s/perf: perf interrupt does not have to get_user_pages to access user memory

read_user_stack_slow that walks user address translation by hand is
only required on hash, because a hash fault can not be serviced from
"NMI" context (to avoid re-entering the hash code) so the user stack
can be mapped into Linux page tables but not accessible by the CPU.

Radix MMU mode does not have this restriction. A page fault failure
would indicate the page is not accessible via get_user_pages either,
so avoid this on radix.

Signed-off-by: Nicholas Piggin <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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