efi/efi_test: Fix uninitialized variable 'datasize'

efi/efi_test: Fix uninitialized variable 'datasize'

Fix minor issue found by CoverityScan:

  CID 1358931 (#1 of 1): Uninitialized scalar variable (UNINIT)9.
  uninit_use: Using uninitialized value datasize.
  199        prev_datasize = datasize;
  200        status = efi.get_variable(name, vd, at, dz, data);

Signed-off-by: Ivan Hu <ivan.hu@canonical.com>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20161018143318.15673-5-matt@codeblueprint.co.uk
Signed-off-by: Ingo Molnar <mingo@kernel.org>