selftests: tls: add a test for splicing onto a full plaintext record

selftests: tls: add a test for splicing onto a full plaintext record

Splicing onto a plaintext sk_msg ring that is already full used to wrap the
ring and make the kernel oops in the scatterwalk once the record was
pushed.

Only the copy path leaves the ring full without pushing it, so splice until
the ring is one fragment short, add the last fragment with a one-byte
MSG_MORE send, and splice once more before pushing the record.

CONFIG_MAX_SKB_FRAGS is 17..45, so that last fragment follows between 16
and 44 splices; sweep that range to trigger the bug on any build.

Signed-off-by: chanyoung <ppoo1220@gmail.com>
Link: https://patch.msgid.link/20260804052837.49015-3-ppoo1220@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>