target/iscsi: Modify iscsit_do_crypto_hash_buf() prototype

Change the type of the last two arguments from u8 * into const void *
and void * respectively such that the u8 * casts can be left out
from the callers. This patch does not change any functionality.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Mike Christie <mchristi@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>