objtool/klp: Fix vmlinux klp relocations for EXPORT_SYMBOL_FOR_MODULES()

When a module function references a vmlinux symbol which is exported
with EXPORT_SYMBOL_FOR_MODULES(), a patch to that function needs to use
a klp reloc.

Currently, livepatch fails to load such a module:

  livepatch: invalid access to vmlinux symbol 'get_task_policy' from module-specific livepatch relocation section
  livepatch: failed to initialize patch 'livepatch_test' for module 'testmod' (-22)
  livepatch: patch 'livepatch_test' failed for module 'testmod', refusing to load module 'testmod'

klp diff puts all klp relocs in __klp_relocs.<patched object>, so
post-link names the section .klp.rela.<patched object>.<secname>, which the
kernel rejects for vmlinux symbols.

Commit 07f14d6af9d77 ("objtool/klp: Fix cross-module klp relocation
section naming") changed the meaning of objname in the klp rela section
name to be where the referenced symbol is referenced rather than where
it lives.  That premise only holds for symbols in a module: the relocs
get applied when the patched module gets patched, and the module
dependency guarantees the referenced module is loaded by then.

A vmlinux symbol needs the opposite.  It's always resolvable, and it has
to be applied when the patch module loads, before the module loader
initializes the patch module's special sections, which may reference it.
That's why livepatch rejects vmlinux symbols in module-specific
sections.

Use "vmlinux" as the section objname when the referenced symbol lives in
vmlinux.  This moves such klp relocs from .klp.rela.kvm..text to
.klp.rela.vmlinux..text.

Fixes: 07f14d6af9d77 ("objtool/klp: Fix cross-module klp relocation section naming")
Reported-by: Dylan Hatch <dylanbhatch@google.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Link: https://patch.msgid.link/f8e3b9fae109903a6aafb2a33310e4afdcebf58e.1786761327.git.jpoimboe@kernel.org
Closes: https://lore.kernel.org/CADBMgpz7iWC0=t=_gE-tfvv0mTPq4kg0qQ2zgPH8DVPE6eQ9Kw@mail.gmail.com

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