tracing: Don't use out-of-sync va_list in event printing

If trace_seq becomes full, trace_seq_vprintf() no longer consumes
arguments from va_list, making va_list out of sync with format
processing by trace_check_vprintf().

This causes va_arg() in trace_check_vprintf() to return wrong
positional argument, which results into a WARN_ON_ONCE() hit.

ftrace_stress_test from LTP triggers this situation.

Fix it by explicitly avoiding further use if va_list at the point
when it's consistency can no longer be guaranteed.

Link: https://lkml.kernel.org/r/[email protected]

Signed-off-by: Nikita Yushchenko <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

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