efi: vars: Switch to new wrapper layer

Switch the caching linked-list efivars layer implementation to the newly
introduced efivar get/set variable wrappers, instead of accessing the
lock and the ops pointer directly. This will permit us to move this code
out of the public efivars API, and into efivarfs once the obsolete sysfs
access method is finally removed.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>