efivars: Check size of user object

Unbelieavably there are no checks to see whether the data structure
passed to 'new_var' and 'del_var' is the size that we expect. Let's add
some for better robustness.

Cc: Mike Waychison <mikew@google.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

diff --cc drivers/firmware/efi/efivars.c
Simple merge