ABI: refuse non-matching ABI minor version on event registration

ABI: refuse non-matching ABI minor version on event registration

In scenarios where a lttng-tools 2.8 (lttng-ust 2.8) stack is running
and an application linked against a lttng-ust 2.7, event registration
will fail on fields size validation [1]. This is not expected based on
the ABI versioning exposed by lttng-ust 2.7 (6.0) and lttng-ust 2.8
(6.1).

The same happen if the scenario is reversed.

This is the result of a change in _ustctl_basic_type.

2.8 introduced enumeration to _ustctl_basic_type. The defined padding is
of 296 while the new union member is 312 (310 of real data + 2 for
alignment) pushing the structure size to 312 instead of the previous
296. This should have been an major ABI break but until now the problem
did not surface.

To prevent this, refuse non matching minor version. No need to check for
particular major,minor version since only 6.0 (ust 2.7) and 6.1 (ust
2.8) exist until a major ABI break.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>