net/tls(TLS_SW): Add selftest for 'chunked' sendfile test

net/tls(TLS_SW): Add selftest for 'chunked' sendfile test

This selftest tests for cases where sendfile's 'count'
parameter is provided with a size greater than the intended
file size.

Motivation: When sendfile is provided with 'count' parameter
value that is greater than the size of the file, kTLS example
fails to send the file correctly. Last chunk of the file is
not sent, and the data integrity is compromised.
The reason is that the last chunk has MSG_MORE flag set
because of which it gets added to pending records, but is
not pushed.
Note that if user space were to send SSL_shutdown control
message, pending records would get flushed and the issue
would not happen. So a shutdown control message following
sendfile can mask the issue.

Signed-off-by: Pooja Trivedi <pooja.trivedi@stackpath.com>
Signed-off-by: Mallesham Jatharkonda <mallesham.jatharkonda@oneconvergence.com>
Signed-off-by: Josh Tway <josh.tway@stackpath.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>