modpost: fix extable entry size calculation.

modpost: fix extable entry size calculation.

As Guenter pointed out, we were never really calculating the extable entry
size because the pointer arithmetic was simply wrong.  We want to check
we're handling the second relocation in __ex_table to infer an entry size,
but we were using (void*) pointers instead of Elf_Rel[a]* ones.

This fixes the problem by moving that check in the caller (since we can
deal with different types of relocations) and add is_second_extable_reloc()
to make the whole thing more readable.

Signed-off-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
CC: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>