powerpc/module_64: Fix "expected nop" error on module re-patching

When a module with a livepatched function is unloaded and then reloaded,
klp attempts to dynamically re-patch it.  On ppc64, that fails with the
following error:

  module_64: livepatch_nfsd: Expected nop after call, got e8410018 at e_show+0x60/0x548 [livepatch_nfsd]
  livepatch: failed to initialize patch 'livepatch_nfsd' for module 'nfsd' (-8)
  livepatch: patch 'livepatch_nfsd' failed for module 'nfsd', refusing to load module 'nfsd'

The error happens because the restore r2 instruction had already
previously been written into the klp module's replacement function when
the original function was patched the first time.  So the instruction
wasn't a nop as expected.

When the restore r2 instruction has already been patched in, detect that
and skip the warning and the instruction write.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/2f6329ffd9674df6ff57e03edeb2ca54414770ab.1674617130.git.jpoimboe@kernel.org

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