vhost/crypto: fix build with GCC 12

vhost/crypto: fix build with GCC 12

GCC 12 raises the following warning:

In file included from ../lib/mempool/rte_mempool.h:46,
                 from ../lib/mbuf/rte_mbuf.h:38,
                 from ../lib/vhost/vhost_crypto.c:7:
../lib/vhost/vhost_crypto.c: In function ‘rte_vhost_crypto_fetch_requests’:
../lib/eal/x86/include/rte_memcpy.h:371:9: warning: array subscript 1 is
     outside array bounds of ‘struct virtio_crypto_op_data_req[1]’
     [-Warray-bounds]
  371 | rte_mov32((uint8_t *)dst + 3 * 32, (const uint8_t *)src + 3 * 32);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../lib/vhost/vhost_crypto.c:1178:42: note: while referencing ‘req’
 1178 |         struct virtio_crypto_op_data_req req;
      |                                          ^~~

Split this function and separate the per descriptor copy.
This makes the code clearer, and the compiler happier.

Note: logs for errors have been moved to callers to avoid duplicates.

Fixes: 3c79609fda7c ("vhost/crypto: handle virtually non-contiguous buffers")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>