net: tls: fix device-offloaded sendpage straddling records

Adrien reports that incorrect data is transmitted when a single
page straddles multiple records. We would transmit the same
data in all iterations of the loop.

Reported-by: Adrien Moulin <amoulin@corp.free.fr>
Link: https://lore.kernel.org/all/61481278.42813558.1677845235112.JavaMail.zimbra@corp.free.fr
Fixes: c1318b3 ("tls: Add opt-in zerocopy mode of sendfile()")
Tested-by: Adrien Moulin <amoulin@corp.free.fr>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Acked-by: Maxim Mikityanskiy <maxtram95@gmail.com>
Link: https://lore.kernel.org/r/20230304192610.3818098-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/tls/tls_device.c

@@ -508,6 +508,8 @@ static int tls_push_data(struct sock *sk,
 			zc_pfrag.offset = iter_offset.offset;
 			zc_pfrag.size = copy;
 			tls_append_frag(record, &zc_pfrag, copy);
+
+			iter_offset.offset += copy;
 		} else if (copy) {
 			copy = min_t(size_t, copy, pfrag->size - pfrag->offset);