selftests: tls: cover splice after a failed decrypt

selftests: tls: cover splice after a failed decrypt

Nothing in this file splices a socket whose last decrypt failed, so
the check that fails tls_sw_splice_read() on a broken connection can
be removed without a test noticing. Such a splice hands the
application plaintext that recvmsg() and read_sock() already refuse
to return.

Extend the bad_auth pattern. Corrupt an authenticated record, confirm
recvmsg() reports EBADMSG, then splice the same socket and require
EBADMSG again. A synchronous decrypt fails again on the still-queued
record, so only an async decrypt reaches EBADMSG through the
recorded-failure check alone.

bad_auth builds the same corrupted record, so its construction moves
into a helper the two tests share.

Signed-off-by: Chuck Lever <cel@kernel.org>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20260806-tls-splice-crypto-fix-v1-2-a2624005a286@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>