mptcp: only set DATA_FIN when a mapping is present

mptcp: only set DATA_FIN when a mapping is present

mptcp_get_options() clears only the status group of struct
mptcp_options_received; data_seq, subflow_seq and data_len are filled in
by mptcp_parse_option() exclusively inside the DSS mapping block, which
runs only when the DSS M (mapping present) bit is set.

A peer can send a DSS option with the DATA_FIN flag set but the mapping
bit clear. The parser then records mp_opt->data_fin while leaving
data_len and data_seq uninitialized. For a zero-length segment
mptcp_incoming_options() evaluates

	if (mp_opt.data_fin && mp_opt.data_len == 1 &&
	    mptcp_update_rcv_data_fin(msk, mp_opt.data_seq, mp_opt.dsn64))

which reads the uninitialized data_len and data_seq; KMSAN reports an
uninit-value in mptcp_incoming_options(). The stale data_seq can also be
fed into the receive-side DATA_FIN sequence tracking.

Record the DATA_FIN flag only when the DSS option carries a mapping, so
data_fin is never set without data_seq and data_len also being present.
data_fin is part of the status group that mptcp_get_options() clears up
front, so on the no-map path it stays zero and the zero-length DATA_FIN
branch is simply skipped. A DATA_FIN is always transmitted together with
a mapping (mptcp_write_data_fin() sets use_map along with data_seq and
data_len), so legitimate DATA_FIN handling is unaffected.

Move the pr_debug() that logs the parsed DSS flags below the mapping
block, so it reports the final data_fin value instead of the stale one
it would otherwise print before the assignment.

Fixes: 43b54c6ee382 ("mptcp: Use full MPTCP-level disconnect state machine")
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260709191925.2811195-1-michael.bommarito@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>