tracing/filter: Do not WARN on operand count going below zero

tracing/filter: Do not WARN on operand count going below zero

When testing the fix for the trace filter, I could not come up with
a scenario where the operand count goes below zero, so I added a
WARN_ON_ONCE(cnt < 0) to the logic. But there is legitimate case
that it can happen (although the filter would be wrong).

 # echo '>' > /sys/kernel/debug/events/ext4/ext4_truncate_exit/filter

That is, a single operation without any operands will hit the path
where the WARN_ON_ONCE() can trigger. Although this is harmless,
and the filter is reported as a error. But instead of spitting out
a warning to the kernel dmesg, just fail nicely and report it via
the proper channels.

Link: http://lkml.kernel.org/r/[email protected]
Reported-by: Vince Weaver <[email protected]>
Reported-by: Sasha Levin <[email protected]>
Cc: [email protected] # 2.6.33+
Signed-off-by: Steven Rostedt <[email protected]>