loader/efi/linux: Use proper type for len variable

Although the length should not exceed 2^31 grub_size_t is more
suitable for that variable. len is used to compute the size
of buffers which in C is a size_t, not a int. It is used
for GRUB_EFI_BYTES_TO_PAGES which expects unsigned values.
It is assigned to load_options_size which is unsigned, not signed.

Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/loader/efi/linux.c

@@ -190,7 +190,7 @@ grub_arch_efi_linux_boot_image (grub_addr_t addr, grub_size_t size, char *args)
   grub_efi_boot_services_t *b;
   grub_efi_status_t status;
   grub_efi_loaded_image_t *loaded_image;
-  int len;
+  grub_size_t len;
   grub_size_t args_len;
 
   mempath = grub_malloc (2 * sizeof (grub_efi_memory_mapped_device_path_t));