Fix: sessiond: check for lttng-modules ABI 2.1 rather than 2.8

Fix: sessiond: check for lttng-modules ABI 2.1 rather than 2.8

The clear patchset introduces a regression that breaks session
rotations when the kernel domain is used. The 2.8 lttng-modules ABI
does not exist yet.

The packet sequence number functionality was introduced in LTTng 2.8,
which introduced the 2.1 kernel tracer ABI.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ief97e7e25f6f0fcb5b5b97b39abb417c1eb327ec