modpost: squash extable_mismatch_handler() into default_mismatch_handler()

Merging these two reduces several lines of code. The extable section
mismatch is already distinguished by EXTABLE_TO_NON_TEXT.

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