blk-crypto: drop the NULL check from blk_crypto_put_keyslot()

Now that all callers of blk_crypto_put_keyslot() check for NULL before
calling it, there is no need for blk_crypto_put_keyslot() to do the NULL
check itself.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20230315183907.53675-2-ebiggers@kernel.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>