tools/slabinfo-gnuplot: force to use bash shell

tools/slabinfo-gnuplot: force to use bash shell

On some linux distributions, the default link of sh is dash which
deoesn't support split array like "${var//,/ }"

It's better to force to use bash shell directly.

Link: http://lkml.kernel.org/r/20171208093751.GA175471@sofia
Signed-off-by: Liu Changcheng <changcheng.liu@intel.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>