MIPS: Ignore relocs against __ex_table for relocatable kernel

Since commit 6f2c2f9 ("scripts/sorttable: Remove unneeded
Elf_Rel"), sorttable no longer clears relocs against __ex_table,
claiming "it was never used."  But in fact MIPS relocatable kernel had
been implicitly depending on this behavior, so after this commit the
MIPS relocatable kernel has started to spit oops like:

	CPU 1 Unable to handle kernel paging request at virtual address 000000fffbbdbff8, epc == ffffffff818f9a6c, ra == ffffffff813ad7d0
	... ...
	Call Trace:
	[<ffffffff818f9a6c>] __raw_copy_from_user+0x48/0x2fc
	[<ffffffff813ad7d0>] cp_statx+0x1a0/0x1e0
	[<ffffffff813ae528>] do_statx_fd+0xa8/0x118
	[<ffffffff813ae670>] sys_statx+0xd8/0xf8
	[<ffffffff81156cc8>] syscall_common+0x34/0x58

So ignore those relocs on our own to fix the issue.

Fixes: 6f2c2f9 ("scripts/sorttable: Remove unneeded Elf_Rel")
Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/boot/tools/relocs.c

@@ -468,6 +468,8 @@ static void walk_relocs(int (*process)(struct section *sec, Elf_Rel *rel,
 			Elf_Sym *sym, const char *symname))
 {
 	int i;
+	struct section *extab_sec = sec_lookup("__ex_table");
+	int extab_index = extab_sec ? extab_sec - secs : -1;
 
 	/* Walk through the relocations */
 	for (i = 0; i < ehdr.e_shnum; i++) {
@@ -480,6 +482,9 @@ static void walk_relocs(int (*process)(struct section *sec, Elf_Rel *rel,
 		if (sec->shdr.sh_type != SHT_REL_TYPE)
 			continue;
 
+		if (sec->shdr.sh_info == extab_index)
+			continue;
+
 		sec_symtab  = sec->link;
 		sec_applies = &secs[sec->shdr.sh_info];
 		if (!(sec_applies->shdr.sh_flags & SHF_ALLOC))