modpost: Get proper section index by get_secindex() instead of st_shndx

(uint16_t) st_shndx is limited to 65535(i.e. SHN_XINDEX) so sym_get_data() gets
wrong section index by st_shndx if requested symbol contains extended section
index that is more than 65535.  In this case, we need to get proper section index
by .symtab_shndx section.

Module.symvers generated by building kernel with "-ffunction-sections -fdata-sections"
shows the issue.

Fixes: 56067812 ("kbuild: modversions: add infrastructure for emitting relative CRCs")
Fixes: e84f9fbb ("modpost: refactor namespace_from_kstrtabns() to not hard-code section name")
Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/mod/modpost.c

@@ -308,7 +308,8 @@ static const char *sec_name(struct elf_info *elf, int secindex)
 
 static void *sym_get_data(const struct elf_info *info, const Elf_Sym *sym)
 {
-	Elf_Shdr *sechdr = &info->sechdrs[sym->st_shndx];
+	unsigned int secindex = get_secindex(info, sym);
+	Elf_Shdr *sechdr = &info->sechdrs[secindex];
 	unsigned long offset;
 
 	offset = sym->st_value;