efivarfs: move variable lifetime management into the inodes

Make the inodes the default management vehicle for struct
efivar_entry, so they are now all freed automatically if the file is
removed and on unmount in kill_litter_super().  Remove the now
superfluous iterator to free the entries after kill_litter_super().

Also fixes a bug where some entry freeing was missing causing efivarfs
to leak memory.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>