kern/efi/mm: Change grub_efi_allocate_pages_real() to call semantically correct free...

kern/efi/mm: Change grub_efi_allocate_pages_real() to call semantically correct free function

If the firmware happens to return 0 as an address of allocated pages,
grub_efi_allocate_pages_real() tries to allocate a new set of pages,
and then free the ones at address 0.

However at that point grub_efi_store_alloc() wasn't yet called, so
freeing the pages at 0 using grub_efi_free_pages() which calls
grub_efi_drop_alloc() isn't necessary, so let's call b->free_pages()
instead.

The call to grub_efi_drop_alloc() doesn't seem particularly harmful,
because it seems to do nothing if the allocation it is asked to drop
isn't on the list, but the call to it is obviously unnecessary here.

Signed-off-by: Mate Kukri <mate.kukri@canonical.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>