MIPS: boot/compressed: Use array reference for image bounds

As done with other image addresses in other architectures, use an
explicit flexible array instead of "address of char", which can trip
bounds checking done by the compiler. Found when building with
-Warray-bounds:

In file included from ./include/linux/byteorder/little_endian.h:5,
from ./arch/mips/include/uapi/asm/byteorder.h:15,
from ./arch/mips/include/asm/bitops.h:21,
from ./include/linux/bitops.h:33,
from ./include/linux/kernel.h:22,
from arch/mips/boot/compressed/decompress.c:13:
arch/mips/boot/compressed/decompress.c: In function 'decompress_kernel':
./include/asm-generic/unaligned.h:14:8: warning: array subscript -1 is outside array bounds of 'unsigned char[1]' [-Warray-bounds]
14 | __pptr->x; \
| ~~~~~~^~~
./include/uapi/linux/byteorder/little_endian.h:35:51: note: in definition of macro '__le32_to_cpu'
35 | #define __le32_to_cpu(x) ((__force __u32)(__le32)(x))
| ^
./include/asm-generic/unaligned.h:32:21: note: in expansion of macro '__get_unaligned_t'
32 | return le32_to_cpu(__get_unaligned_t(__le32, p));
| ^~~~~~~~~~~~~~~~~
arch/mips/boot/compressed/decompress.c:29:37: note: while referencing '__image_end'
29 | extern unsigned char __image_begin, __image_end;
| ^~~~~~~~~~~

Cc: Thomas Bogendoerfer <[e-mail]>
Cc: Randy Dunlap <[e-mail]>
Cc: [e-mail]
Signed-off-by: Kees Cook <[e-mail]>
Reviewed-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>

arch/mips/boot/compressed/decompress.c

···
 unsigned long free_mem_end_ptr;
 
 /* The linker tells us where the image is. */
-extern unsigned char __image_begin, __image_end;
+extern unsigned char __image_begin[], __image_end[];
 
 /* debug interfaces  */
 #ifdef CONFIG_DEBUG_ZBOOT
···
 {
 	unsigned long zimage_start, zimage_size;
 
-	zimage_start = (unsigned long)(&__image_begin);
-	zimage_size = (unsigned long)(&__image_end) -
-	    (unsigned long)(&__image_begin);
+	zimage_start = (unsigned long)(__image_begin);
+	zimage_size = (unsigned long)(__image_end) -
+	    (unsigned long)(__image_begin);
 
 	puts("zimage at:     ");
 	puthex(zimage_start);
···
 		dtb_size = fdt_totalsize((void *)&__appended_dtb);
 
 		/* last four bytes is always image size in little endian */
-		image_size = get_unaligned_le32((void *)&__image_end - 4);
+		image_size = get_unaligned_le32((void *)__image_end - 4);
 
 		/* The device tree's address must be properly aligned  */
 		image_size = ALIGN(image_size, STRUCT_ALIGNMENT);