Documentation/trace/postprocess/trace-vmscan-postprocess.pl: fix the traceevent regex

When irq, preempt and lockdep fields are printed (field 3 in the example
below) in the trace output, the script fails.

An example entry:
  kswapd0-610   [000] ...1   158.112152: mm_vmscan_kswapd_wake: nid=0 order=0

Signed-off-by: Vinayak Menon <vinayakm.list@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>