staging: lttng: Fix recent modifications to string_from_user operation

Include: a fix for a recently introduced change: obviously max_t should
be used instead of min_t here. Also, a likely should apply to the result
of the comparison, not the variable per se.

Signed-off-by: Yannick Brosseau <yannick.brosseau@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>