efivarfs: Ensure VariableName is NUL-terminated

Some buggy firmware implementations update VariableNameSize on success
such that it does not include the final NUL character which results in
garbage in the efivarfs name entries.  Use kzalloc on the efivar_entry
(as is done in efivars.c) to ensure that the name is always
NUL-terminated.

The buggy firmware is:
BIOS Information
        Vendor: Intel Corp.
        Version: S1200RP.86B.02.02.0005.102320140911
        Release Date: 10/23/2014
        BIOS Revision: 4.6
System Information
        Manufacturer: Intel Corporation
        Product Name: S1200RP_SE

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Matthew Garrett <mjg59@coreos.com>
Cc: Jeremy Kerr <jk@ozlabs.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>