tracing: fix return value in __ftrace_event_enable_disable for TRACE_REG_UNREGISTER

tracing: fix return value in __ftrace_event_enable_disable for TRACE_REG_UNREGISTER

[ Upstream commit 0c588ac0ca6c22b774d9ad4a6594681fdfa57d9d ]

When __ftrace_event_enable_disable invokes the class callback to
unregister the event, the return value is not reported up to the
caller, hence leading to event unregister failures being silently
ignored.

This patch assigns the ret variable to the invocation of the
event unregister callback, so that its return value is stored
and reported to the caller, and it raises a warning in case
of error.

Link: https://lore.kernel.org/20250321170821.101403-1-gpaoloni@redhat.com
Signed-off-by: Gabriele Paoloni <gpaoloni@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>