efivarfs: Move efivar availability check into FS context init

Instead of checking whether or not EFI variables are available when
creating the superblock, check it one step earlier, when initializing
the FS context for the mount. This way, no FS context will be created at
all, and we can drop the second check at .kill_sb() time entirely.

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