tls: prevent oversized sendfile() hangs by ignoring MSG_MORE

We got multiple reports that multi_chunk_sendfile test
case from tls selftest fails. This was sort of expected,
as the original fix was never applied (see it in the first
Link:). The test in question uses sendfile() with count
larger than the size of the underlying file. This will
make splice set MSG_MORE on all sendpage calls, meaning
TLS will never close and flush the last partial record.

Eric seem to have addressed a similar problem in
commit 35f9c09f

 ("tcp: tcp_sendpages() should call tcp_push() once")
by introducing MSG_SENDPAGE_NOTLAST. Unlike MSG_MORE
MSG_SENDPAGE_NOTLAST is not set on the last call
of a "pipefull" of data (PIPE_DEF_BUFFERS == 16,
so every 16 pages or whenever we run out of data).

Having a break every 16 pages should be fine, TLS
can pack exactly 4 pages into a record, so for
aligned reads there should be no difference,
unaligned may see one extra record per sendpage().

Sticking to TCP semantics seems preferable to modifying
splice, but we can revisit it if real life scenarios
show a regression.

Reported-by: Vadim Fedorenko <vfedorenko@novek.ru>
Reported-by: Seth Forshee <seth.forshee@canonical.com>
Link: https://lore.kernel.org/netdev/1591392508-14592-1-git-send-email-pooja.trivedi@stackpath.com/
Fixes: 3c4d7559

 ("tls: kernel TLS support")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Tested-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>