modpost: squash report_extable_warnings() into extable_mismatch_handler()

Collect relevant code into one place to clarify all the cases are
covered by 'if () ... else if ... else ...'.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>