efivarfs: Use sizeof() instead of magic number

Instead of adding a magic 4 to the variable size, use sizeof() to make
it explicitly clear what the quantity represents (the variable's
attributes).

CC: Jeremy Kerr <jeremy.kerr@canonical.com>
Cc: Chun-Yi Lee <joeyli.kernel@gmail.com>
Cc: Andy Whitcroft <apw@canonical.com>
Reported-by: Lingzhu Xiang <lxiang@redhat.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>