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

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>