efi/arm: Fix boot crash with CONFIG_CPUMASK_OFFSTACK=y

On ARM and arm64, we use a dedicated mm_struct to map the UEFI
Runtime Services regions, which allows us to map those regions
on demand, and in a way that is guaranteed to be compatible
with incoming kernels across kexec.

As it turns out, we don't fully initialize the mm_struct in the
same way as process mm_structs are initialized on fork(), which
results in the following crash on ARM if CONFIG_CPUMASK_OFFSTACK=y
is enabled:

  ...
  EFI Variables Facility v0.08 2004-May-17
  Unable to handle kernel NULL pointer dereference at virtual address 00000000
  [...]
  Process swapper/0 (pid: 1)
  ...
  __memzero()
  check_and_switch_context()
  virt_efi_get_next_variable()
  efivar_init()
  efivars_sysfs_init()
  do_one_initcall()
  ...

This is due to a missing call to mm_init_cpumask(), so add it.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: <stable@vger.kernel.org> # v4.5+
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/1488395154-29786-1-git-send-email-ard.biesheuvel@linaro.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

drivers/firmware/efi/arm-runtime.c

@@ -65,6 +65,7 @@ static bool __init efi_virtmap_init(void)
 	bool systab_found;
 
 	efi_mm.pgd = pgd_alloc(&efi_mm);
+	mm_init_cpumask(&efi_mm);
 	init_new_context(NULL, &efi_mm);
 
 	systab_found = false;