kmsan: silence -Wmissing-prototypes warnings

kmsan: silence -Wmissing-prototypes warnings

When building the kernel with W=1, the compiler reports numerous warnings
about the missing prototypes for KMSAN instrumentation hooks.

Because these functions are not supposed to be called explicitly by the
kernel code (calls to them are emitted by the compiler), they do not have
to be declared in the headers.  Instead, we add forward declarations right
before the definitions to silence the warnings produced by
-Wmissing-prototypes.

Link: https://lkml.kernel.org/r/20230112103147.382416-1-glider@google.com
Signed-off-by: Alexander Potapenko <glider@google.com>
Reported-by: Vlastimil Babka <vbabka@suse.cz>
Suggested-by: Marco Elver <elver@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/202301020356.dFruA4I5-lkp@intel.com/T/
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>