mptcp: use plain bool instead of custom binary enum

mptcp: use plain bool instead of custom binary enum

The 'data_avail' subflow field is already used as plain boolean,
drop the custom binary enum type and switch to bool.

No functional changed intended.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231023-send-net-next-20231023-2-v1-3-9dc60939d371@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>