efivarfs: Make 'datasize' unsigned long

There's no reason to declare 'datasize' as an int, since the majority
of the functions it's passed to expect an unsigned long anyway. Plus,
this way we avoid any sign problems during arithmetic.

Acked-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>