modpost: remove fromsym info in __ex_table section mismatch warning

report_extable_warnings() prints "from" in a pretty form, but we know
it is always located in the __ex_table section, i.e. a collection of
struct exception_table_entry.

It is very likely to fail to get the symbol name and ends up with
meaningless message:

  ... in reference from the (unknown reference) (unknown) to ...

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>