stacktrace: Get rid of unneeded '!!' pattern

My commit b0c51f15

 ("stacktrace: Don't skip first entry on
noncurrent tasks") adds one or zero to skipnr by "!!(current == tsk)".

But the C99 standard says:

  The == (equal to) and != (not equal to) operators are
  ...
  Each of the operators yields 1 if the specified relation is true and 0
  if it is false.

So there is no need to prepend the above expression by "!!" -- remove it.

Reported-by: Joe Perches <joe@perches.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191111092647.27419-1-jslaby@suse.cz
Signed-off-by: Ingo Molnar <mingo@kernel.org>