efivars: only check for duplicates on the registered list

efivars: only check for duplicates on the registered list

variable_is_present() accesses '__efivars' directly, but when called via
gsmi_init() Michel reports observing the following crash,

  BUG: unable to handle kernel NULL pointer dereference at (null)
  IP: variable_is_present+0x55/0x170
  Call Trace:
    register_efivars+0x106/0x370
    gsmi_init+0x2ad/0x3da
    do_one_initcall+0x3f/0x170

The reason for the crash is that '__efivars' hasn't been initialised nor
has it been registered with register_efivars() by the time the google
EFI SMI driver runs.  The gsmi code uses its own struct efivars, and
therefore, a different variable list.  Fix the above crash by passing
the registered struct efivars to variable_is_present(), so that we
traverse the correct list.

Reported-by: Michel Lespinasse <[email protected]>
Tested-by: Michel Lespinasse <[email protected]>
Cc: Mike Waychison <[email protected]>
Cc: Matthew Garrett <[email protected]>
Cc: Seiji Aguchi <[email protected]>
Signed-off-by: Matt Fleming <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>