modpost: refactor sech_name()

Use sym_get_data_by_offset() helper to get access to the .shstrtab
section data. No functional change is intended because
elf->sechdrs[elf->secindex_strings].sh_addr is 0 for both ET_REL
and ET_EXEC object types.

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