efivars: Make efivars bin_attributes dynamic

In preparation for encapsulating efivars, we need to have the
bin_attributes be dynamically allocated so that we can use their
->private fields to get back to the struct efivars structure.

Signed-off-by: Mike Waychison <mikew@google.com>
Cc: Matt Domsch <Matt_Domsch@dell.com>,
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>