efi/x86: Use non-blocking SetVariable() for efi_delete_dummy_variable()

efi/x86: Use non-blocking SetVariable() for efi_delete_dummy_variable()

Presently, efi_delete_dummy_variable() uses set_variable() which might
block, which the scheduler is rightfully upset about when used from
the idle thread, producing this splat:

  "bad: scheduling from the idle thread!"

So, make efi_delete_dummy_variable() use set_variable_nonblocking(),
which, as the name suggests, doesn't block.

Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@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/20180711094040.12506-3-ard.biesheuvel@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>