iscsi-target: Make buf param of iscsit_do_crypto_hash_buf() const void *

Make the "buf" input param of iscsit_do_crypto_hash_buf() "const void *".
This allows to remove lots of casts in its callers.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>