: echo: write error: Invalid argument

[ Impact: fix to allow negative operand in filer expr ]

Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
LKML-Reference: <4A35B8DF.60400@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
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