treewide: Drop WARN_ON_FUNCTION_MISMATCH

CONFIG_CFI_CLANG no longer breaks cross-module function address
equality, which makes WARN_ON_FUNCTION_MISMATCH unnecessary. Remove
the definition and switch back to WARN_ON_ONCE.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Kees Cook <keescook@chromium.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220908215504.3686827-15-samitolvanen@google.com