splice, net: Fix SPLICE_F_MORE signalling in splice_direct_to_actor()

splice_direct_to_actor() doesn't manage SPLICE_F_MORE correctly[1] - and,
as a result, it incorrectly signals/fails to signal MSG_MORE when splicing
to a socket.  The problem I'm seeing happens when a short splice occurs
because we got a short read due to hitting the EOF on a file: as the length
read (read_len) is less than the remaining size to be spliced (len),
SPLICE_F_MORE (and thus MSG_MORE) is set.

The issue is that, for the moment, we have no way to know *why* the short
read occurred and so can't make a good decision on whether we *should* keep
MSG_MORE set.

MSG_SENDPAGE_NOTLAST was added to work around this, but that is also set
incorrectly under some circumstances - for example if a short read fills a
single pipe_buffer, but the next read would return more (seqfile can do
this).

This was observed with the multi_chunk_sendfile tests in the tls kselftest
program.  Some of those tests would hang and time out when the last chunk...