efivarfs: add variable resync after hibernation

Hibernation allows other OSs to boot and thus the variable state might
be altered by the time the hibernation image is resumed.  Resync the
variable state by looping over all the dentries and update the size
(in case of alteration) delete any which no-longer exist.  Finally,
loop over all efi variables creating any which don't have
corresponding dentries.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
[ardb: - apply error pointer fixup from Dan Carpenter
       - rebase onto latest version of James's efivarfs rework]
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>