block: blk-crypto-fallback: remove redundant initialization of variable err

The variable err is being initialized with a value that is never read
and it is being updated later with a new value.  The initialization is
redundant and can be removed.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Satya Tangirala <satyat@google.com>
Addresses-Coverity: ("Unused value")
Signed-off-by: Jens Axboe <axboe@kernel.dk>